Clinical Potential of the Acyclic Nucleoside Phosphonates Cidofovir, Adefovir, and Tenofovir in Treatment of DNA Virus and Retrovirus Infections

CMV retinitis.

In the study reported by Lalezari et al. (119), cidofovir was found to be efficacious in delaying the progression of CMV retinitis (in patients with AIDS) when given intravenously at 5 mg/kg once weekly for 2 weeks (induction therapy) followed by once every other week (maintenance therapy). Treatment was associated with manageable side effects. Strict adherence to monitoring of renal function before cidofovir was administered and concomitant administration of probenecid and saline hydration appeared to minimize drug-related nephrotoxicity (119). In the SOCA/ACTG trial (209a), cidofovir at two treatment regimens (induction therapy, as indicated above; maintenance therapy at 5 mg/kg every other week [high dose] or 3 mg/kg every other week [low dose]) effectively slowed the progression of CMV retinitis (in patients with AIDS), with the high-dose regimen being clearly more efficacious than the low-dose regimen (209a). Further follow-up over a long-term period (210) showed similar rates of progression and median times to progression with both low maintenance and high maintenance doses of cidofovir to those reported in the initial study (210). From the long-term follow-up study (21), it was concluded that cidofovir is effective for the treatment of CMV retinitis but that it has potential for toxicity (both renal and ocular), which typically resolves on discontinuation of therapy.

The efficacy of cidofovir therapy for AIDS-associated CMV retinitis has been confirmed in patients receiving highly active antiretroviral therapy (HAART), although in this study a higher incidence of iritis was noted, probably as a consequence of the patients' enhanced ability to mount an inflammatory response (21). In fact, anterior uveitis occurs after a mean of 8.5 intravenous infusions of cidofovir (52); it responds to treatment with topical corticosteroids and mydriatics and does not preclude the continuation of cidofovir therapy (3) unless ocular hypotony develops (13).

To assess the effect of intravenous cidofovir on delaying the progression of previously treated, relapsing CMV retinitis, a randomized, controlled comparison of two maintenance doses of cidofovir was conducted: AIDS patients with CMV retinitis that had progressed despite treatment with ganciclovir, foscarnet, or both, were randomized to receive induction cidofovir (5 mg/kg once weekly for 2 weeks) and then maintenance therapy with either 5 mg/kg or 3 mg/kg once every other week (118). The median time to retinitis progression as assessed by retinal photography was not reached in the 5-mg/kg dose group and was 49 days in the 3-mg/kg dose group (118).

Recent studies have indicated that the regimens of, on the one hand, ganciclovir implantation (i.e., a surgically placed intraocular implant) plus oral ganciclovir and, on the other hand, intravenous cidofovir are equally effective in slowing the progression of CMV retinitis (Fig. 4) and preventing visual loss (Fig. 5) (211). A phaseI study indicated that combination therapy of intravenous cidofovir (5 mg/kg intravenously every 2 weeks) with oral ganciclovir (1 g orally three times a day) may further enhance clinical efficacy (104).

Cidofovir has also proved efficacious, on intravitreal injection, against CMV retinitis (in patients with AIDS) (112, 113), although intravitreal injection of HPMPC should be used with caution because of the risk of anterior uveitis and decrease of intraocular pressure (15).

CMV disease.

In clinical trials cidofovir has proven efficacious not only against CMV retinitis but also against other manifestations of CMV disease, e.g., intravenously against asymptomatic CMV infection in HIV-infected patients (121, 165). Cidofovir can be recommended as a preemptive treatment for CMV disease (i.e., treatment initiated if CMV antigen or DNA is detected in the blood) after allogeneic blood stem cell transplantation (163). In the study reported by Platzbecker et al. (163), 9 (90%) of 10 patients showed a response to cidofovir treatment, with 7 of the 9 experiencing a complete clearance of the virus (pp65 negative, PCR negative); treatment-related toxicity was moderate, with 4 patients developing reversible renal impairment.

In another study (129), half of the patients who were treated for CMV disease after allogeneic stem cell transplantation responded to cidofovir therapy, as did 66% of the patients who had failed or relapsed after previous preemptive therapy with ganciclovir or foscarnet and 62% of the patients in whom cidofovir was used as the primary preemptive therapy (129). Similar results were obtained in another prospective study of primary preemptive therapy with cidofovir (31).

Cidofovir should also be considered as second-line therapy in patients with CMV disease failing to respond to ganciclovir or foscarnet (54, 129). According to a case report (28), cidofovir effected a complete resolution of CMV retinitis, CMV encephalitis, and CMV esophagitis after only 2 months of intravenous therapy and after initial attempts to stop the disease with ganciclovir and foscarnet had been unsuccessful.

Herpesvirus infections other than CMV infections.

Cidofovir has proven efficacious not only against CMV disease but also against HSV infections, e.g., when injected intravenously (115, 122) or applied topically (120, 195) to treat acyclovir-resistant mucocutaneous HSV infections. Such acyclovir-resistant HSV infections may be quite severe in immunosuppressed (e.g., AIDS) patients. As described in a recent case report, a severe perianal HSV-2 infection in an AIDS patient that was refractory to both ganciclovir and acyclovir treatment healed virtually completely within 30 days of treatment with intravenous cidofovir at a dose of 5 mg/kg once weekly for a total of 3 weeks (115). In a child with AIDS presenting with a facial HSV ulcer that had developed resistance to acyclovir and did not respond to foscavir either, local application of cidofovir 1% led to a prompt recovery (123).

The safety and efficacy of single topical applications of 1, 3, and 5% cidofovir gel have been monitored in a multicenter double-blind, randomized phase I/II dose-escalating study for the treatment of early, lesional, recurrent genital herpes at five Canadian outpatient sites (178). Cidofovir gel at all strengths significantly decreased the median time to negative virus culture: 3.0 days in the placebo group versus 2.2, 1.3, and 1.1 days in the 1, 3, and 5% cidofovir gel treatment groups, respectively (Fig. 6). Application site reactions occurred in 3, 5, 19, and 22% of the patients in these four groups, respectively. Additional studies are warranted to further identify the optimal efficacious dose of cidofovir that can be tolerated (178).

The emergence of acyclovir- and/or foscarnet-resistant HSV-1 infections after allogeneic stem cell transplantation has become an increasing concern (38); since these HSV strains retain sensitivity to cidofovir, the use of cidofovir in such cases should be advocated. In fact, in a specific case of a foscarnet- and acyclovir-resistant HSV infection after an unrelated bone marrow transplantation for a relapse of acute myeloblastic leukemia, cidofovir effected spectacular improvements after each intravenous dose (5 mg/kg) and complete healing of the herpetic lesions after the seventh injection (29). In another patient, who had undergone umbilical cord stem cell transplantation and who had severe unremitting HSV-1 mucositis of the oropharynx, which was unresponsive to acyclovir and foscarnet therapy, the mucositis cleared after three consecutive once-weekly doses of cidofovir (intravenously at 5 mg/kg once weekly) and the patient was able to tolerate oral nutrition (130).

There is anecdotal evidence for the efficacy of cidofovir in the treatment of oral hairy leukoplakia (which is presumably due to EBV infection) (121). According to a case report, treatment with cidofovir together with rituximab (anti-CD20 monoclonal antibody) led to complete remission of an EBV-associated lymphoma involving the central nervous system (95). Also, in another case of EBV-associated lymphoma, intralesional injections of cidofovir led to a marked regression of the lesions (A. Meerbach, M. Schacke, P. Hyckel, H. Kosmehl, and P. Wutzler, Abstr. 15th Int. Conf. Antiviral Res., abstr. 91, 2002). Cidofovir may also be expected to be effective in the adjunctive treatment of nasopharyngeal carcinoma since it strongly inhibits the growth of nasopharyngeal carcinoma xenografts in nude mice, apparently through the induction of apoptosis (142, 154).

Whether cidofovir would be effective in the treatment of KS máy depend on the interplay of different factors. In two patients with AIDS, an important regression of all cutaneous KS lesions was noted after 3 months of cidofovir treatment (5 mg/kg intravenously at 1-week intervals for the first two injections and every 2 weeks thereafter) (136). A clinical response of KS lesions to intravenous cidofovir treatment has also been noted in an HIV-negative homosexual man (84). In another patient without AIDS, classical KS was not affected by intralesional injections of cidofovir (186). Successful treatment of KS in a HIV-negative man was observed with intravenous cidofovir treatment added to liposomal daunorubicin (12). Abatement of cutaneous KS was noted in a patient with AIDS and CMV retinitis who was treated with cidofovir (94). Since opportunistic infections such as those due to CMV may trigger the development of KS through the release of cytokines and growth factor, inhibition of CMV replication by cidofovir may contribute to the abatement of KS lesions (185).

Adenovirus infections.

Adenovirus infection can be particularly severe in allogeneic stem cell transplant recipients. Intravenous cidofovir (with concomitant probenecid) at a dose of 5 mg/kg/week for 2 weeks and then every 2 weeks for a total of five doses proved successful in suppressing the manifestations of adenovirus disease in an allogeneic stem cell transplant recipient (174).

In five children who had developed a systemic adenovirus infection after bone marrow transplantation for leukemia, cidofovir (administered intravenously at 5 mg/kg once weekly for 3 weeks, then every 2 weeks) effected clinical improvement of diarrhea, cystitis, and fever and a concomitant disappearance of the virus, as assessed by PCR and cell culture (124).

From a retrospective analysis of 35 patients who were identified with adenovirus infection after having undergone allogeneic hematopoietic stem cell transplantation, it appeared that only cidofovir and donor leukocyte infusion were effective options whereas ribavirin and vidarabine were not (30).

In a prospective trial initiated to evaluate cidofovir in the treatment of adenovirus infections in hematopoietic stem cell transplant recipients, eight patients were enrolled on a dosage schedule of 1 mg/kg (intravenously) three times weekly (101). All of these patients eventually achieved long-term viral suppression and clinical improvement, although six patients needed prolonged cidofovir therapy for up to 8 months before the cidofovir administration could be stopped without adenovirus recurrence; no dose-limiting nephrotoxicity was observed, and discontinuation of the drug was not required in any patients (101).

Although topical cidofovir has proven to be effective against adenovirus and HSV keratoconjunctivitis in animal models (79, 108, 175-177), it has not been intensively pursued for the corresponding indications in humans. There is an anecdotal report of the potential effectiveness of 0.2% cidofovir eye drops in a patient with adenoviral conjunctivitis (91), but a larger study failed to show a statistically significant effect of topical 0.2% cidofovir on the course of the acute phase of adenoviral conjunctivitis (98). Eye drops of 1% cidofovir did prove effective in the prevention of severe corneal opacities and, concomitantly, caused inflammation of the eyelids and conjunctiva (99). This local toxicity is probably related to the too frequent administration (4 or 10 times daily) of the 1% cidofovir eye drops; further clinical studies are needed to delineate the optimal treatment regimen for adenovirus keratoconjunctivitis.

Polyomavirus infections.

There are a vast number of reports pointing to the efficacy of cidofovir (given at the dosage regimen recommended for the treatment of CMV retinitis in AIDS patients) in the treatment of PML in AIDS patients. For a series of 12 patients with AIDS-associated PML, a significant correlation was found between JC virus load and survival time (206). In one case, the JC virus load in CSF, which can be considered a prognostic parameter for the clinical outcome of PML, decreased and then became undetectable after cidofovir treatment; this was associated with clinical improvement (206). This patient had been receiving HAART for 2 months without experiencing a modification in JC virus load, before the cidofovir therapy was started (206).

In HIV-infected hosts, the clinical course of PML is almost invariably fatal and there is no proven effective therapy for this condition. Initial anecdotal reports of responses to cytarabine (cytosine arabinoside) have not been confirmed in larger or randomized trials (78). If, however, cytarabine was combined with cidofovir, a remarkable turnaround of PML was noted in a patient with advanced HIV disease (27); cidofovir may well have been the major factor contributing to the successful resolution of PML in this patient (27).

In a multicenter study of consecutive HIV-positive patients with histologically or virologically proven PML, 26 patients were treated with HAART only while 14 patients were treated with HAART plus cidofovir (5 mg/kg intravenously per week for the first 2 weeks and alternate weeks thereafter) (77). Cidofovir added to HAART was associated with a more effective control of JC virus replication, with improved neurological outcome and survival compared with that due to HAART alone (Fig. 7) (77).

Substantial clinical, virological, and neuroradiological improvement has also been reported in various other AIDS patients with PML following the addition of cidofovir to the HAART regimen (32, 75, 166, 170, 232). This suggests that the combination of HAART with cidofovir improves the outcome of AIDS-related PML (232). For a typical example of the neuroradiological improvement following cidofovir therapy, see Fig. 8 (75). Also, the efficacy of cidofovir in the treatment of JC virus-associated PML has been demonstrated in a patient with systemic lupus erythematosus (179).

In a patient with idiopathic CD4⁺ lymphocytopenia and PML, cidofovir was found to be apparently inefficacious (93). However, in this particular case (93), it was not demonstrated whether the disease was actually caused by, or associated with, JC virus (93). Similarly, no clinical benefit was noted with cidofovir in the treatment of PML in a child with hyperimmunoglobulin E recurrent infection syndrome (9), although in this patient a CSF sample obtained after the first two cidofovir infusions was still positive for JC virus DNA (9).

In a follow-up study of the multicenter observational study mentioned above (77), it was ascertained that the 1-year cumulative probability of survival was 0.61 with cidofovir and 0.29 without. After adjusting for baseline CD4 counts, the JC virus load in the CSF, Karnofsky score, and use of HAART prior to the onset of PML, the use of cidofovir was independently associated with a reduced risk of death (76).

In another monocenter observational study of the effect of cidofovir on AIDS-associated PML, the 1-year cumulative probability of being active was 62% in the cidofovir-plus-HAART group, compared to 53% in the HAART group (87). An additional benefit with respect to survival was observed in patients who were given cidofovir after adjustment to the baseline variables CSF JC virus load, CD4 cell count, and expanded disability status scale (87).

Although there is overwhelming evidence to suggest that cidofovir offers at least partial benefit in the treatment of JC virus-associated PML (see above), its role in the treatment of BK polyomavirus-associated hemorrhagic cystitis is less well established. In a patient with BK polyomavirus-associated acute hemorrhagic cystitis who had received an allogeneic bone marrow transplant, cidofovir proved efficacious (90), but in another case of BK virus-associated hemorrhagic cystitis in an HIV-infected patient, there was apparently no response to cidofovir (17). BK virus-associated nephropathy has been increasingly recognized as an important cause of renal transplant dysfunction, and cidofovir (at a dose as low as 0.25 to 1 mg/kg) was found to reduce BK viruria to undetectable levels, accompanied by stable renal function for 6 to 26 months post-therapy.

HPV infections.

Complete and permanent remissions of papillomatous lesions have been achieved following either topical gel application or direct intralesional injections of cidofovir. The first case of a hypopharyngeal/esophageal papilloma (due to HPV-16) that showed complete regression after topical cidofovir therapy was reported several years ago (Fig. 9) (216).

The efficacy of cidofovir in the topical treatment (as a 1% gel) of anogenital HPV-16 infections was first shown in three AIDS patients with severe relapsing penile, perigenital, intra-anal or cervical/vulvar condylomata (198); following topical cidofovir treatment, the lesions disappeared, and the patients remained free of disease during the 6- to 12-month follow-up period (198). Recalcitrant condyloma, unresponsive to any other form of treatment, disappeared after topical application of cidofovir 1.5% in a viscous gel (97); when applied topically at 1% in amphiphile cream once daily for 2 weeks, cidofovir effected a virtually total disappearance of extensive penile condyloma acuminata (181). A randomized, placebo-controlled trial indicated that 1% cidofovir cream was significantly more effective than vehicle cream in the treatment of external anogenital warts in HIV-infected patients (135).

In another double-blind, placebo-controlled, study of 1% cidofovir gel in the treatment of anogenital HPV infections, a partial to complete response was observed in 84.2% of the cidofovir-treated patients compared to 18.2% of the placebo-treated patients (197).

An open randomized prospective study indicated that topical 1% cidofovir gel was more effective than electrocautery in preventing recurrences of genital warts in HIV-infected patients (relapse rates, 35.29 and 73.68%, respectively) and that the two procedures, if combined, effected a complete response (158).

Initial clinical trials with 1% cidofovir topical gel also point to its efficacy in the treatment of cervical intraepithelial neoplasia grade III (196): partial to complete responses were observed histologically in the majority of the patients and were confirmed by PCR, and this effect was seen after only three applications (every other day) (196). Complete and permanent eradication of vulvar intraepithelial neoplasia grade III was achieved with topical 1% cidofovir in a patient who did not respond to interferon and isotretinoin (114). Also, bowenoid papulosis (perianal intraepithelial neoplasia) in an AIDS patient was found to respond to cidofovir: after three treatment cycles (0.4% cidofovir cream twice daily for five consecutive days every 15 weeks), the lesions completely disappeared (80). A similar, complete cure was seen in an AIDS patient with bowenoid papulosis of the penis after three rounds of 1% cidofovir cream applications (once daily for 5 days) (Fig. 10) (201).

The potential of cidofovir to treat of laryngeal papillomatosis has been clearly demonstrated by studies of a number of patients with severe recurrent laryngeal papillomatosis (200), where local intralesional injections of cidofovir (diluted in saline at 2.5 mg/ml) caused a complete and durable regression of the papillomatous lesions in the majority of the patients (Fig. 11). Although minor papilloma recurrences may arise following cidofovir treatment, these recurrences seem to respond to additional intralesional injections, so that eventually complete cures are achieved (225). Percutaneous injection of cidofovir has been recommended for office-based treatment of adult patients with anterior laryngeal papillomatosis, since it reduces the need for repeated direct laryngoscopy and laser ablation under general anesthesia (42). Intralesional cidofovir therapy has been advocated as an excellent treatment option for laryngeal papillomas in adults, although it requires perseverance from both the patient and the surgeon to achieve remission of the disease (22).

The potential benefit of intralesional administration of cidofovir (2.5 mg/ml) has also been assessed in children with severe recurrent respiratory papillomatosis: they demonstrated a dramatic response at the follow-up visit 9 months after the last injection of cidofovir (i.e., complete regression of the tumor after 15 injections every 2 to 3 weeks) (Fig. 12) (167), which led the authors (167, 168) to conclude that intralesional injection of cidofovir may be of benefit in the treatment of severe respiratory papillomatosis in pediatric patients.

From three case studies, it now appears that systemic (intravenous) cidofovir, whether combined with alpha interferon or used alone, would also be effective against recurrent respiratory papillomatosis with involvement of the lung parenchyma (11, 58, 221); under these conditions, laryngeal and tracheal lesions may regress or even disappear and pulmonary lesions may undergo consolidation.

Cidofovir has also proved successful in the topical treatment of verruca vulgaris: two patients with a 2- to 3-year history of recurrent verruca vulgaris that did not respond to repeated conventional therapies (electrodesiccation, liquid nitrogen, cryosurgery, CO₂ laser ablation, etc.) experienced complete disappearance of the verruca vulgaris lesions following once- or twice-daily therapy with 3% topical cidofovir (229). These patients have remained completely free of lesions for more than 40 weeks (230). A large plantar wart (caused by HPV-66) in an AIDS patient completely resolved within 3 to 4 weeks following two courses of topical 3% cidofovir therapy (Fig. 13) (60).

In another AIDS patient, recalcitrant HPV gingival infection, which did not respond to conventional therapies, healed within 2 weeks after treatment with 1% cidofovir cream (once daily for 5 consecutive days, for 2 weeks) (35). Topical cidofovir (1% cream) has proved effective in the treatment of cutaneous HPV lesions, refractory to conventional methods, in a number of patients (203), including patients with AIDS (36). Inflammation and/or erosion may occur at the site of application of the cidofovir cream (36). While such erosions invariably heal and may actually reflect an effective response of the HPV lesions to cidofovir treatment, care should be taken not to apply cidofovir on abraded skin. Also, for topical application, a concentration of 1% (wt/wt) cidofovir should not be exceeded. If applied at too high a concentration (e.g., 4%) over a too large surface of abraded skin for too long a period (daily for 12 days), there is a risk of systemic toxicity, as illustrated by a case of acute renal failure in a bone marrow transplant recipient with chronic renal failure who developed genital condylomas and was treated with topical 4% cidofovir for 12 days (23).

In view of the circumstantial evidence for the effectiveness of cidofovir in the treatment of virtually all HPV-associated diseases, it is intriguing that in one particular patient with epidermodysplasia verruciformis, topical and systemic treatment of cidofovir had no apparent effect (169). On the other hand, cidofovir injected intralesionally proved successful in the treatment of squamous cell carcinoma, a disease that is supposedly not related to HPV (37). This antineoplastic activity may be based on the same mechanism, i.e., induction of apoptosis, that underlies the inhibitory effect of cidofovir on the growth of HPV-associated tumors.

Poxvirus infections.

Cidofovir is effective in the treatment of molluscum contagiosum, a cutaneous skin growth caused by the poxvirus MCV. Recalcitrant molluscum contagiosum in three AIDS patients resolved completely and permanently after either topical cidofovir treatment (3% cream, applied once daily) or intravenous cidofovir treatment (5 mg/kg each week for 2 weeks, followed by 5 mg/kg once every 2 weeks) (137). A similar complete and permanent remission of recalcitrant molluscum contagiosum was seen in an AIDS patient following nine cycles of intravenous cidofovir treatment (5 mg/kg once every 2 weeks) (Fig. 14) (103). Similar complete and durable resolution of molluscum contagiosum lesions was noted in children following topical treatment with cidofovir (at either 1% or 3%), e.g., in a boy with Wiskott-Aldrich syndrome (59), in two otherwise healthy children (231), and in two HIV-infected children (212).

We recently reported the case of a 39-year-old renal transplant patient (under immunosuppression with cyclosporine, methylprednisolone, and mycophenolate mofetil) who developed a giant orf (ecthyma contagiosum) lesion, which continued to grow to dramatic proportions (88). Topical treatment with 1% cidofovir cream (five cycles of 5 days with and 5 days without treatment) effected a complete resolution, with only granulation tissue left; after some signs of recurrence, the lesion was treated with another two courses of cidofovir cream, after which a complete cure ensued (Fig. 15) (88).

Cidofovir has not been clinically used in the treatment of poxvirus infections other than orf or molluscum contagiosum. Based on the data obtained with cidofovir in the treatment of vaccinia virus and cowpox virus infections in experimental animal model infections (33, 34, 149, 188-190), cidofovir may be expected to be effective in the therapy as well as the pre- and postexposure prophylaxis of smallpox, monkeypox, and vaccinia virus infections in humans, whether administered intravenously or perorally (in its oral prodrug form [HDP-CDV]) or aerosolized (against an aerosolized virus infection) (68).

Should smallpox vaccination, based on the use of the live vaccinia virus vaccine, be reinstalled, it would be formally contraindicated to use this vaccine in immunocompromised patients, whatever the cause of their immunodeficiency (primary immune deficiency, HIV infection, immunosuppressive therapy, etc.). Inadvertent use of the live vaccinia virus vaccine in such patients may lead to a serious, life-threatening, disseminated, and progressive vaccinia (111, 171). The therapy or prophylaxis of such complications may well represent a primary indication for the use of cidofovir.

HIV infections.

Adefovir dipivoxil has been initially pursued for the treatment of HIV infections. In a randomized, double-blind, placebo-controlled dose escalation study (16), adefovir dipivoxil, at three dose levels (125, 250, or 500 mg once daily) was found to effect a significant decrease in serum p24 antigen levels and HIV RNA copy numbers. Adefovir dipivoxil was best tolerated at the lowest dose studied (125 mg daily), and its anti-HIV activity was already evident after 8 days of dosing (16). In another randomized, double-blind, placebo-controlled study (71), adefovir dipivoxil at the two dose levels studied (125 mg and 250 mg) was found to effect a significant decrease in HIV-1 RNA plasma load and a significant increase in CD4⁺ T-cell counts throughout the 12 weeks of treatment. The drug was determined to be safe and well tolerated when administered for 12 weeks (71).

Single-dose pharmacokinetics and the safety of oral adefovir dipivoxil have also been established in children infected with HIV-1 (102), and the results of this study were quoted as providing sufficient information to warrant proceeding to a multidose phase II study to further evaluate the safety, pharmacokinetics, and efficacy of adefovir dipivoxil in children. Hughes et al. (102) noted that in children the oral clearance (CL/F) of adefovir (dipivoxil) was greater than in adults (when a dose of 3.0 mg/kg in children was compared with a 60-mg dose in adults).

In one 24-week, randomized, double-blind, placebo-controlled multicenter study, adefovir dipivoxil, administered as a single daily oral dose of 120 mg added to stable antiretroviral therapy, effected a 0.4-log₁₀/ml decline from baseline in the HIV RNA plasma load, compared with no change in the placebo group (Fig. 16) (107). This reduction in viral load extended beyond 24 weeks but was accompanied by (reversible) nephrotoxicity (107). In another randomized, double-blind, placebo-controlled multicenter trial, no virologic or immunologic benefit was observed when oral adefovir dipivoxil (120 mg once daily) was added to background antiretroviral therapy in treating advanced HIV disease (85). The difference in the virologic results between the two studies (85, 107) may be explained by the fact that the patients in the latter study (85) had more advanced disease (lower CD4⁺ cell counts) and were more drug experienced than those in the former study (107). In the latter study (85), the use of adefovir dipivoxil (120 mg daily) was associated with considerable nephrotoxicity.

HIV-infected patients harboring the lamivudine-associated RT M184V mutation showed the largest decline in viral load (almost −1.0 log₁₀ unit from baseline) (157), which seems to relate to the fact that the M184V mutation results in increased in vitro susceptibility to adefovir (140).

Combination therapy with adefovir dipivoxil plus efavirenz, in highly treatment-experienced patients, resulted in a marked viral load suppression (>2.0 log₁₀ units at 24 weeks) but only in patients who had no prior experience with nonnucleoside RT inhibitors (184).

In view of the rather modest reduction in viral load observed with adefovir dipivoxil (only a 0.4-log₁₀/ml decline in viral load [Fig. 16] [107]) and the accompanying risk for nephrotoxicity (development of a Fanconi-like renal syndrome consisting of acidosis, proteinuria, glucosuria, hypophosphatemia, and elevated creatinine levels at the dosage used [120 mg once daily]), adefovir dipivoxil has not further been pursued for the treatment of HIV infections.

HBV infections.

Adefovir dipivoxil has been actively pursued for the therapy of chronic hepatitis B. In a placebo-controlled phase I/II study, adefovir dipivoxil given orally at 125 mg as a single daily dose for 28 days effected a 1.8-log₁₀ pg/ml reduction in HBV DNA levels, compared to an increase of 0.01 log₁₀ pg/ml in the control patients with chronic HBV infection (89).

Lamivudine, the current drug of choice for the treatment of chronic hepatitis B, has been shown to elicit resistance in 20, 38, 49, 66, and 69 of patients after 1, 2, 3, 4, or 5 years of lamivudine therapy, respectively (C. E. Westland, C. S. Gibbs, M. D. Miller, M. Sullivan, J. Fry, C. L. Brosgart, M. Wulfsohn, and S. Xiong, Oral Presentations 37th Annu. Meet. Eur. Assoc. Study Liver Diseases, abstr. 568, 2002). A 48-week, randomized, double-blind, placebo-controlled, multicenter study was designed to evaluate the efficacy of adefovir dipivoxil in patients with chronic hepatitis B due to lamivudine-resistant HBV (M. Peters, H. W. Hann, P. Martin, E. Heathcote, P. Buggisch, A. E. Moorat, M. Sullivan, K. Kleber, R. Ebrahimi, S. Xiong, and S. Brosgart, Oral Presentations 37th Annu. Meet. Eur. Assoc. Study Liver Dis., abstr. 646, 2002). Patients were randomized to receive either adefovir dipivoxil (10 mg), or adefovir dipivoxil (10 mg) plus lamivudine (100 mg), or lamivudine (100 mg). After 16 weeks, patients in both the adefovir dipivoxil monotherapy and combination adefovir dipivoxil plus lamivudine groups achieved a similar reduction in serum HBV DNA levels from baseline (2.86 and 2.87 log₁₀ copies per ml), compared to no change in patients treated with 100-mg lamivudine monotherapy (Fig. 17). After 32 weeks of treatment, 32% of the patients who had received adefovir dipivoxil monotherapy had lost the lamivudine resistance mutation M204V/I (located within the YMDD motif).

An open-label pilot study with oral adefovir dipivoxil (10 mg once daily) in patients coinfected with HIV-1 and lamivudine-resistant HBV indicated significant declines in serum HBV DNA concentrations from baseline (8.64 log₁₀ copies per ml): −3.40 log₁₀ copies per ml at week 24 and −4.01 log₁₀ copies per ml at week 48 (Fig. 18) (20). This treatment was well tolerated and did not produce significant side effects, except for a transient increase in alanine aminotransferase (ALT) levels in some patients. The concomitant use of adefovir dipivoxil and antiretroviral regimens in these HIV-1/HBV-coinfected patients did not influence either HIV-1 replication or the development of HIV-1 mutations (20).

Successful treatment with adefovir dipivoxil (10 mg once daily, orally) has been reported in several anecdotal cases of lamivudine-resistant HBV infections (161), including patients with fulminant hepatic failure (162) or fibrosing cholestatic hepatitis (223). Acute liver graft failure due to the emergence of lamivudine-resistant HBV promptly resolved during treatment with adefovir dipivoxil (143).

In patients with HBV e antigen (HBeAg)-positive chronic hepatitis B, 48 weeks of 10 or 30 mg of adefovir dipivoxil per day resulted in histologic liver improvement, reduced serum HBV DNA levels, reduced serum ALT levels, and increased rates of HBeAg seroconversion. Since there was a higher frequency of adverse events and renal abnormalities in the group given 30 mg of adefovir dipivoxil per day, the 10-mg dose was considered to have the better risk-benefit profile for long-term treatment (132).

Adefovir dipivoxil has also been the subject of a double-blind, placebo-controlled, multicenter phase III clinical trial in patients with precore mutant (e-antigen-negative) HBV patients (92). Patients were randomized to receive either 10 mg of adefovir dipivoxil or placebo once daily for 48 weeks. Treatment with adefovir dipivoxil resulted in a median reduction in the serum HBV DNA level from baseline of 3.91 log₁₀ copies per ml, compared with a median reduction of 1.35 log₁₀ copies per ml in patients who received placebo (Fig. 19). ALT levels normalized in 72% of the adefovir dipivoxil patients compared to 29% in the placebo group. Of the patients treated with adefovir dipivoxil, 64% exhibited significant improvement in liver histology, compared to 33% of the patients on placebo.

cccDNA is a key intermediate in HBV replication is and considered the reservoir responsible for the persistence of chronic HBV infection. Treatment with adefovir dipivoxil reduced the level of HBV cccDNA in the liver by 89% from baseline at 48 weeks, whereas placebo treatment resulted in no reduction (B. Werle, K. Wursthorn, J. Petersen, S. Bowden, S. Locarnini, C. James, C. Brosgart, S. Xiong, W. Delaney, C. Gibbs, and F. Zoulim, Oral Presentations 37th Annu. Meet. Eur. Assoc. Study Liver Dis., abstr. 638, 2002). Preliminary data also point to the regression of cirrhosis is 5 of 11 patients following adefovir dipivoxil treatment, compared to 0 of 12 patients following placebo treatment (P. Marcellin, Z. Goodman, T. T. Chang, S. G. Lim, M. Tong, W. Sievert, M. Schiffman, L. Jeffers, M. Wulfsohn, R. Fallis, J. Fry, and C. Brosgart, Oral Presentations 37th Annu. Meet. Eur. Assoc. Study Liver Dise. abstr. 560, 2002).

The antiviral efficacy of adefovir dipivoxil in hepatitis B patients is independent of the HBV genotype: after 48 weeks, treatment with adefovir dipivoxil (10 mg once daily, orally) resulted in a significant decrease in serum HBV DNA levels across all HBV genotypes (A, B, C, D, E, F, and G), with mean reductions ranging from 3.4 to 4.2 log₁₀ copies per ml (C. Westland, W. Delaney, H. Yang, J. Fry, C. Brosgart, C. Gibbs, M. Miller, and S. Xiong, Poster Presentations 37th Annu. Meet. Eur. Assoc. Study Liver Diseases, abstr. 273, 2002).

As an extension to the open-label pilot study in patients coinfected with HIV-1 and lamivudine-resistant HBV (20), these patients were further treated with adefovir dipivoxil (10 mg once daily, orally) beyond 48 weeks (Y. Benhamou, M. Bochet, V. Thibault, V. Calvez, M. H. Fievet, M. Sullivan, C. Brosgart, H. Namini, T. Poynard, and C. Katlama, Abstr. 9th Conf. Retroviruses Opportunistic Infect. abstr. 123, 2002; Y. Benhamou, M. Bochet, V. Thibault, V. Calvez, M. H. Fievet, M. Sullivan, C. Brosgart, H. Namini, T. Poynard, and C. Katlama, Poster Presentations 37th Annu. Meet. Eur. Assoc. Study Liver Dis., abstr. 245, 2002). The mean decrease in serum HBV DNA from baseline (8.64 log₁₀ copies per ml) was −3.40 log₁₀ copies per ml at week 48, −4.77 log₁₀ copies per ml at week 72, and −5.13 log₁₀ copies per ml at week 92 (Fig. 20). Following long-term therapy with adefovir dipivoxil for chronic hepatitis B, HBV DNA was undetectable (<400 copies per ml) by week 100 in 70% of the patients (Fig. 21) (E. J. Heathcote, L. Jeffers, R. Perrillo, T. Wright, M. Sherman, H. Namini, S. Xiong, C. James, V. Ho, J. Fry, and C. Brosgart, Poster Presentations 37th Annu. Meet. Eur. Assoc. Study Liver Dis., abstr. 294, 2002). Over the course of the study, 21% of the patients achieved seroconversion of the HBV e antigen.

Adefovir dipivoxil has also been evaluated for treatment of lamivudine-resistant HBV infection of patients with liver failure requiring liver transplantation and patients with chronic hepatitis after liver transplantation. Although not placebo controlled, adefovir dipivoxil treatment was associated with impressive and favorable evolution of the clinical, virological, and biochemical parameters. In all studies, adefovir dipivoxil proved safe and was well tolerated. At a dose of 10 mg/day, significant renal abnormalities were not observed (D. Mutimer, Abstract, Antiviral Ther. 7:L84, 2002).

The safety and efficacy of adefovir dipivoxil were further assessed in a study of 40 patients (26 without and 14 with liver transplantation) with decompensated chronic hepatitis B, YMDD variant, and reduced clinical and virological response to lamivudine. Addition of adefovir dipivoxil to ongoing lamivudine therapy in these patients was well tolerated and led to significant inhibition of viral replication, and improvement of both clinical status and biochemical parameters (such as ALT levels) (D. Mutimer, H.-W. L. Hann, M. Buti, S. Strasser, K. Watkins, M. Woessner, C. Brosgart, E. Bourne, D. Tait, and R. Perrillo, Abstract, Antiviral Ther. 7:L90, 2002).

Adefovir dipivoxil can be considered a safe drug for the treatment of chronic HBV infections: an integrated analysis of two phase III studies with HBeAg⁺ and HBeAg⁻ patients revealed a safety profile for adefovir dipivoxil at 10 mg/day that was similar to that of placebo throughout 48 weeks; there was no evidence of renal laboratory abnormalities in either the adefovir dipivoxil or placebo groups (E. J. Heathcote, T. T. Chang, S. G. Lim, S. Hadziyannis, N. Tassopoulos, M. Tong, W. Sievert, M. Wollman, S. Van Doren, J. Fry, and C. Brosgart, Abstract, Antiviral Ther. 7: L105, 2002).

While there has been general concern in the past about the potential nephrotoxicity of adefovir dipivoxil when used at a daily dose of 60 mg (and, a fortiori, at 120 mg), it should be pointed out that in the pivotal phase III clinical trial involving 172 HBV-infected patients, 93% of whom were treated with 10 mg of adefovir dipivoxil daily for 48 weeks, there was a mean reduction in viral titer of 3.52 log₁₀ units and a mean reduction in the ALT level of 49 IU/liter, with no patients having creatinine level increases of >0.5 mg/dl or phosphorus levels of <1.5 mg/dl (18). Benhamou et al. (20) have supported these findings and furthermore described the complications of interpreting the causes of renal toxicity during multiple treatment regimens.

HIV infections.

In a randomized, double-blind, placebo-controlled, dose escalation clinical trial, intravenous tenofovir monotherapy at two doses (1 and 3 mg/kg/day), administered on days 1 and 8 through 14, effected a significant decline in plasma HIV-1 RNA levels, with the reduction in HIV-1 RNA being sustained for at least 7 days after completion of dosing in the 3-mg/kg/day dose cohort (72). From this study (72) and the pharmacokinetics study in dogs, it was ascertained that tenofovir has a terminal half-life of approximately 10 h and is mainly-excreted unchanged in the urine (57). As with adefovir, several potentially orally bioavailable prodrugs of tenofovir were evaluated for anti-HIV activity, chemical and intestinal stability, and oral bioavailability (10). From a series of alkyl methyl carbamate prodrugs of PMPA, the bis(isopropoxycarboxymethyl)ester of PMPA, termed tenofovir disoproxil, was chosen as the clinical candidate, its oral bioavailability in dogs being 30% (183). Meanwhile, tenofovir disoproxil has been approved and is marketed worldwide as its fumarate salt (Viread; 300 mg once daily) for the treatment of HIV infections.

GS-7340 represents another oral prodrug form of PMPA: it corresponds to 9-[(R)-[[[[(S)-1-(isopropoxycarbonyl)ethyl]amino]phenoxy-phosphinyl]methoxy]propyl]adenine (82): this prodrug of tenofovir, apart from being orally bioavailable, is stable in plasma and selectivity taken up by lymphatic tissue and PBMCs, where it achieves markedly increased levels of tenofovir compared with tenofovir disoproxil (W. Lee, G. He, A. Mulato, W. Delaney, E. Eisenberg, T. Cihlar, S. Xiong, M. Miller, S. Gill, R. Shibata, and C. Gibbs, Abstr. 9th Conf. Retroviruses Opportunistic Infect. abstr. 384-T, 2002).

Tenofovir disoproxil fumarate (tenofovir DF) has been evaluated in a 48-week, randomized, double-blind, placebo-controlled study of treatment-experienced HIV-1-infected patients at three different dose levels, namely, 75, 150, and 300 mg daily (180). After 24 weeks, patients who were initially randomized to placebo were given tenofovir DF (300 mg) for another 24 weeks. In this study by Schooley et al. (180) tenofovir DF provided a dose-related durable reduction in the HIV-1 RNA load, with a safety profile similar to that of placebo.

To further assess the efficacy and safety of tenofovir DF in the treatment of HIV-1 infections, a randomized, double-blind, placebo-controlled phase III study was conducted with antiretroviral-experienced patients, whereby either placebo or 300 mg of tenofovir DF was administered orally once daily for 24 weeks, after which all patients received open-label drug for another 24 weeks (K. Squires, G. Pierone, D. Berger, C. Steinhart, N. Bellos, S. L. Becker, S. S. Chen, M. D. Miller, D. F. Coakley, and A. Cheng, Abstr. 9th Conf. Retroviruses Opportunistic Infect., abstr. 413-W, 2002). Tenofovir DF effected a rapid antiviral response (0.6 log₁₀ unit reduction in viral load) from week 2, which was maintained through week 48 (Fig. 22). This favorable response was also reflected by a significant increase in the percentage of patients with plasma HIV-1 RNA levels of ≤400 or ≤50 copies per ml following treatment with tenofovir DF (Fig. 23). The safety profile of tenofovir DF in this study was similar to that of placebo. The safety and efficacy of tenofovir DF were confirmed in an expanded-access program study conducted in the European Union, United States, Canada, and Australia, where viral load reductions of 1.2 log₁₀ were noted (Fig. 24) (S. Follansbee, J. Reynes, M. Nelson, B. Clotet, A. Lazzarin, A. Adam, S. Van Doren, R. Buffels, S. Barriere, L. Zagury, I. Miranski, and J. Rooney, Abstr. 9th Conf. Retroviruses Opportunistic Infect. abstr. 415-W, 2002).

Adding tenofovir DF (300 mg) to existing antiretroviral therapy for highly treatment-experienced patients with preexisting resistance mutations has proved to achieve significant and durable reductions in HIV-1 RNA levels through week 96. Through 96 weeks of tenofovir DF therapy, there was infrequent development of RT mutations associated with tenofovir DF therapy (K65R mutation, 3%), consistent with the durability of the observed HIV-1 RNA reductions (134).

From an ongoing 3-year, randomized, double-blind, clinical trial being conducted at 81 sites in the United States, Europe, and South America and comparing the efficacy and safety of a treatment regimen of tenofovir DF, lamivudine, and efavirenz to a regimen of stavudine, lamivudine, and efavirenz in 600 antiretroviral-naive patients with HIV infection, the 48-week data and, subsequently, the 96-week data were divulged (S. Staszewski, J. Gallant, A. Pozniak, J. M. A. H. Suleiman, E. Dejesus, E. Koenig, S. Coleman, B. Lu, A. K. Cheng, and D. F. Coakley, Oral Presentations XIV Int. AIDS Conf., abstr. Or17, 2002; S. Staszewski, J. E. Gallant, A. L. Pozniak, J. M. A. H. Suleiman, E. DeJesus, J. Sayre, B. Lu, A. Cheng, Abstr. 10th Conf. Retroviruses Opportunistic Infect., abstr. 564b, 2003). Both treatment groups showed a mean reduction in HIV RNA load of 3.09 log₁₀ copies per ml (baseline, 4.9 log₁₀ copies per ml) after 48 weeks; 95% of the patients in the tenofovir DF arm compared to 91% in the stavudine arm had reductions in HIV RNA levels to below 50 copies/ml after 96 weeks. Thus, the stavudine and tenofovir DF arms showed similar, high efficiencies. However, lipid abnormalities (increase in triglyceride and cholesterol levels) were significantly lower (P < 0.001) in the tenofovir DF arm than in the stavudine arm. Also, the toxicities (peripheral neuropathy, lipodystrophy, lactic acidosis, and pancreatitis) associated with mitochondrial dysfunction through week 96 were markedly lower in the tenofovir DF arm than in the stavudine arm, while the two arms showed a similar renal safety profile. Although this study is still under way, the ad interim results point to a similar efficiency but better safety profile of tenofovir DF compared to stavudine.

Genotypic analyses of HIV samples from patients enrolled in two placebo-controlled clinical trials have demonstrated that most patients who received tenofovir DF in addition to their existing combination antiretroviral regimen achieved significant viral RNA load reductions relative to placebo, regardless of the number and type of thymidine analogue mutations (TAMs) (M41L, D67N, K70R, L210W, T215Y/F, or K219Q/E/N) present in the viral reverse transcriptase (studies 902 and 907; (M. D. Miller, N. A. Margot, A. K. Cheng, B. Lu, Oral Presentations XIV Int. AIDS Conf. abstr. B1390, 2002); changes in HIV RNA levels at 24 weeks were −0.80, −0.66, −067, and −0.21 log₁₀ copy per ml if the number of baseline TAMs was zero, one to two, three or more (without M41L or L210W), or three or more (including M41L or L210W). Even the last patients, with the most advanced pattern of TAM resistance, showed a significant reduction in viral load after treatment with tenofovir DF. Although tenofovir DF may still work in these patients with advanced TAMS, investigators should be striving for treatment approaches that do not lead to such extensive TAM development.

Not only may tenofovir DF be expected to be active in vivo against HIV-1 strains that are resistant to various nucleoside analogues, but also it should be effective against non-B subtypes of HIV-1. Indeed, tenofovir and adefovir were found to be equally active in vitro against a panel of HIV-1 subtypes A, B, C, D, E, F, and G and group O primary HIV-1 isolates (160).

Monotherapy with oral tenofovir DF (300 mg, once daily) in chronically HIV-1-infected antiretroviral-naive individuals has also been studied; after 3 weeks of therapy, a reduction of 1.5 log₁₀ units in plasma HIV-1 RNA levels was noted, an antiviral response that was as robust as previously noted with ritonavir monotherapy (131). These data support further pursuit of tenofovir DF in simplified combinations of antiviral agents as initial treatment for chronic HIV-1 infection.

It should be noted that in comparison with cidofovir, tenofovir has substantially weaker effects on the proliferation and viability of renal proximal tubule epithelial cells (47). It is also less toxic toward erythroid and myeloid progenitor cells than are the nucleoside RT inhibitors zidovudine, stavudine, and zalcitabine (47). Furthermore, tenofovir at concentrations that greatly exceed those required for anti-HIV activity in peripheral blood mononuclear cells is not associated with mitochondrial toxicity (26); this is most probably due to the low efficiency of tenofovir incorporation by mitochondrial DNA polymerase (105) and, again, contrasts with the observations made for the nucleoside analogues zidovudine, stavudine, didanosine, and zalcitabine (26).

HAART is being received by an increasing percentage of pregnant women, pregnancy being of little impediment to its use, despite few published studies regarding the safety of antiretroviral drugs for mother and fetus. Tenofovir was assessed for its safety and efficacy during pregnancy and postnatally in gravid rhesus monkeys whether infected or not infected with SIV (207, 208). Efficient placental transport of tenofovir and significant reduction of viral load in SIV-infected fetuses were shown to result in healthy newborns (208), although the maternal dose used (30 mg/kg/day) throughout gestation transiently affected maternal bone biomarkers and altered some fetal parameters, the long-term ramifications of which need further follow-up (207).

HBV infections.

In patients coinfected with HIV and HBV, a once-daily dose of 300 mg of tenofovir DF effected a reduction in serum HBV DNA levels of 3.34 log₁₀ copies per ml after 12 weeks and of 4.63 log₁₀ copies per ml after 24 weeks. This response was similar for both wild-type and lamivudine-resistant HBV and thus points to the potential of tenofovir as a treatment modality not only for patients infected with HIV but also for those coinfected with HIV and HBV (D. Cooper, A. Cheng, D. Coakley, J. Sayre, L. Zhong, S. S. Chen, C. Westland, M. Miller, and C. Brosgart, Abstr. 9th Conf. Retroviruses Opportunistic Infect., abstr. 124, 2002; M. Bochet, R. Tubiana, Y. Benhamou, V. Thibault, L. Suffisseau, C. Brosgart, J. Rooney, M. Sullivan, T. Poynard, F. Bricaire, and C. Katlama, Abstr. 9th Conf. Retroviruses Opportunistic Infect., abstr. 675-M, 2002).