A Phase I Trial and Pharmacokinetic Study of Sorafenib in Children with Refractory Solid Tumors or Leukemias: A Children’s Oncology Group Phase I Consortium Report

Patient population

Patients ≥24 months and ≤21 years of age with treatment-refractory, histologically confirmed measurable, or evaluable solid extracranial tumors (part A) or with refractory leukemias with >25% leukemic blasts in the bone marrow (part B) were eligible. Other eligibility criteria included ability to swallow intact tablets; recovery from toxic effects of prior therapy; Karnofsky/Lansky performance score ≥50; interval from prior therapy ≥21 days for myelosuppressive chemotherapy or monoclonal antibody treatment; ≥7 days for hematopoietic growth factors, ≥2 weeks for local palliative radiation; ≥3 months from total body, craniospinal, or ≥50% radiation to the pelvis; ≥6 weeks from other substantial bone marrow radiation; ≥3 months from a stem cell transplant or rescue; and no evidence of active graft versus host disease; adequate renal function [age-adjusted normal serum creatinine, or GFR ≥70 mL/min/1.73 m²); and adequate liver function [total bilirubin ≤1.5 × institutional upper limit of normal (ULN), albumin ≥2 g/dL, and alanine aminotransferase (ALT) ≤110 U/L]. Adequate bone marrow function was required and defined as an absolute neutrophil count (ANC) ≥1,000/μL, transfusion-independent platelet count ≥75,000/μL, and hemoglobin ≥8.0 g/dL for part A; and platelet count ≥20,000/μL and hemoglobin ≥8.0 g/dL for part B.

After dose-limiting serum, ALT/aspartate aminotransferase (AST) elevation was observed at the starting dose level in one patient with hepatic metastases and preexisting grade 1 elevation in AST/ALT, the eligibility criteria were revised to require a normal ALT (≤45 U/L) at enrollment. Eligibility criteria were re-revised in part B due to allow for mild elevation in the ALT (≤225 U/L) as many patients with leukemia have disease-related baseline elevations in serum transaminases. Additional eligibility criteria included normal serum lipase and amylase; diastolic blood pressure within the 95th percentile for age and gender and not receiving anti-hypertensive medication. Exclusion criteria included pregnancy or lactation; uncontrolled infection; concurrent use of other investigational agents, anticancer agents, agents to prevent organ rejection posttransplant, cytochrome P450 enzyme-inducing antiepileptic drugs, rifampin, grapefruit juice, St. Johns Wort, or therapeutic anticoagulants; evidence of bleeding diathesis; leptomeningeal disease; and Gilbert syndrome.

This trial was approved by the Institutional Review Boards of participating sites. All patients or their legal guardians signed a document of informed consent indicating their understanding of the investigational nature and risks of this study. Assent was obtained according to institutional guidelines.

Drug administration and study design

Sorafenib was supplied as 50- and 200-mg tablets by the Cancer Therapy Evaluation Program (NCI, Bethesda, MD) and administered orally, twice daily, continuously for 28-day cycles. The starting dose was 150 mg/m²/d with planned escalations to 200, 250, and 325 mg/m²/dose. Doses were prescribed using a dosing nomogram with a maximum dose of 600 mg/dose, as higher doses were intolerable in adults. Patients maintained a diary to document drug intake.

A traditional 3 +3 phase I dose-escalation scheme was used. Intrapatient dose-escalation was not permitted. At the solid tumor MTD, the trial was expanded to enroll 12 evaluable solid tumor patients (6 < 12 and 6 ≥12 years of age) to study steady-state pharmacokinetics and further define sorafenib toxicities (part A), and children with refractory leukemias to evaluate the tolerability, pharmacokinetics, and pharmacodynamics of sorafenib (part B).

Toxicity assessment and disease evaluations

Monitoring for sorafenib-related toxicity included physical examination with blood pressure measurement (weekly during cycle 1, then every other week); weekly serum chemistries including electrolytes, calcium, phosphate, magnesium, creatinine, ALT, bilirubin, lipase, and amylase; total protein and albumin before each cycle; and complete blood counts (twice weekly during cycle 1, then weekly). Clinical and laboratory adverse events, except hypertension, were graded according to the NCI Common Terminology Criteria for Adverse Events version 3 (http://ctep.cancer.gov/protocolDevelopment/electronic_applications/docs/ctcaev3.pdf). Age- and gender-adjusted norms were used to determine whether the blood pressure was elevated (10) and a previously described algorithm for management of sorafenib-related hypertension was used (11).

As inhibition of VEGF has been previously shown to affect bone growth in juvenile animals by thickening of the growth plate and inhibition of trabecular bone formation (12–14), plain radiographs of tibial growth plates were conducted at baseline in patients <18 years of age, and then after every 3 cycles in patients with open growth plates.

Response was evaluated using Response Criteria in Solid Tumors (15) and published criteria for leukemias (16) at baseline and before every odd cycle in part A as well as at the end of cycles 1 and 2, and before every other cycle in part B.

Definition of DLT and MTD

Hematologic DLT was defined as any grade 4 neutropenia (<500/μL) or thrombocytopenia (<25,000/μL). For patients with leukemia, hematologic toxicity was monitored but not used to define the MTD or considered dose-limiting. Non-hematologic DLTs were any sorafenib-related grade 4 toxicity and any grade 3 toxicity with exception of the following: nausea and vomiting of less than 5 days duration, ALT or AST elevations that recovered to grade ≤1 within 7 days of stopping drug, fever, or infection <5 days, and electrolyte abnormalities responsive to oral supplementation. Persistent (≥7 days) grade 2 toxicities intolerable to the patient were also defined as DLTs. Dose-limiting hypertension was initially defined as grade ≥2 hypertension; however, this definition was redefined after dose level 1 accrual to include grade 4 hypertension, a confirmed diastolic blood pressure >25 mm Hg above the ULN, or an elevated diastolic blood pressure not controlled by anti-hypertensive medication within 14 days (11). Patients with a DLT who resolved within 14 days of sorafenib discontinuation were eligible for one dose reduction provided toxicity resolved to meet on study parameters within 14 days. Sorafenib was permanently discontinued for recurrent DLTs.

Patients were considered fully evaluable for toxicity and determination of the MTD provided they either developed a DLT at any time during cycle 1 or did not develop a DLT and received at least 85% of the prescribed sorafenib dose during cycle 1. Patients not meeting these criteria were replaced. The MTD was the dose level immediately below the dose level at which 2 or more patients in a dose level of up to 6 patients experienced a DLT.

Pharmacokinetics and pharmacodynamics

During the dose-escalation phase, pharmacokinetic sampling was conducted in consenting patients after the first dose of sorafenib (day 1). Three milliliters of heparinized blood was collected before and 0.5, 1, 2, 3, 5, 8, 10–12, 24, and 30–36 hours after dose 1 (doses 2 and 3 were held). Trough samples were obtained on days 5, 7, and 27–28 during cycle 1 in all patients. In the expanded solid tumor cohort and the leukemia cohort, sorafenib pharmacokinetics were evaluated at steady state. Blood was collected 12 hours after the last sorafenib dose, which was also used as the level immediately before the next dose, as well as 0.5, 1, 2, 3, 5, and 8 hours after the dose of sorafenib between days 14 and 28 of cycle 1. Sorafenib plasma concentrations were measured using the high-performance liquid chromatography tandem mass spectroscopic (HPLC/MS-MS) method adapted from an assay (17) with a lower limit of quantification of 5 ng/mL (18).

The sorafenib plasma concentration–time data were analyzed using noncompartmental methods. Peak sorafenib concentration (C_max) and time to peak concentration (T_max) were determined from each patient’s concentration–time plot. For day 1, the sorafenib exposure [area under curve (AUC)_0–24h] was calculated using the linear trapezoidal method. For steady-state pharmacokinetics, the pre-sorafenib trough concentration was also used as the 12-hour after sorafenib concentration based on the assumption that at steady state the trough concentration should be constant. At steady state, the AUC over the dosing interval (AUC_0–12h) is equivalent to the AUC_0–∞, if no time-dependent changes occur in exposure. Therefore, apparent clearance was calculated by dividing the sorafenib dose by the AUC_0–12h. The relationship between day 1 and steady-state drug exposure (normalized AUC_0–24h and AUC_0–12h, respectively) to gender, DLT, and response (<4 or ≥4 cycles received) were evaluated by the Mann–Whitney U test. The relationship between clearance and age was evaluated by linear regression.

Blood samples for plasma VEGF, soluble VEGFR2, endoglin, and placental growth factor (PlGF) were obtained in EDTA tubes and quantified at baseline and on days 27 and 28 of cycle 1 using a commercially available ELISA assay (Quantikine, R&D Systems; ref. 19). Total circulating endothelial cells (CEC) and circulating endothelial progenitor cells (CEPC) were analyzed from baseline and day 27 or 28 of cycle 1 blood samples using a 4-color fluorescence-activated cell sorter (19). The average paired values obtained before treatment and at steady state were compared (Student t test) and correlations were made to drug exposure for day 1 and steady-state pharmacokinetics and response for each growth factor analyzed (Spearman correlation).

Patient characteristics

Of 65 patients enrolled from August 2006 to February 2010, 5 were ineligible [required pretreatment lipase not conducted (n = 3), preexisting organ dysfunction (n = 2)] and 10 were not fully evaluable for toxicity: disease progression during cycle 1 (n = 4), best patient interest (n = 3), acute death after 4 days of sorafenib in patient with numerous preexisting conditions (n = 1), non–dose-limiting grade 2 allergic reaction (n = 1), never received sorafenib (n = 1). Table 1 shows characteristics of the 60 eligible patients.

Toxicity and MTD

There were no patient deaths attributed to sorafenib toxicity. Table 2 summarizes cycle 1 DLTs. The MTD was exceeded at the first dose level (150 mg/m²/dose) with DLT in 4 of 6 fully evaluable patients, one of whom developed grade 3 AST/ALT elevation in the presence of preexisting grade 1 transaminase elevation and hepatic metastases, and one of whom developed grade 2 hypertension. No DLTs were observed at the next lower dose (105 mg/m²), which corresponds to a fixed dose of approximately 190 mg, approximately half of the adult recommended dose. The protocol was amended to require normal ALT and bilirubin at enrollment and to provide new guidelines for the grading and management of hypertension (11) as described in the Materials and Methods section. With these changes, sorafenib was re-escalated to an intermediate dose of 130 mg/m² with no DLT in 3 patients, and then to 150, 200, and 250 mg/m², at which dose the MTD was exceeded with DLT (rash/hand foot skin reaction and hyponatremia) in 2 of 2 evaluable patients. The 200 mg/m²/dose was defined as the MTD with DLT (grade 3 ALT) in 1 of 7 patients. In the expanded solid tumor cohort, DLTs were similar to those previously observed and occurred in 1 of 6 patients ≥12 years, and in 2 of 6 patients <12 years old, for a combined DLT rate of 21% (4 of 19) in all evaluable solid tumor patients at the MTD. Four patients developed grade 3 DLTs during subsequent treatment cycles: at 200 mg/m², diarrhea (n = 2), hyponatremia (recurrent DLTs, n = 1); at 150 mg/m², hand foot skin reaction (n = 1). The solid tumor MTD was not tolerated in children with leukemias with DLT (anorexia and gastrointestinal hemorrhage) in 2 of 2 patients, and the 150 mg/m²/dose was determined as the MTD. No dose-limiting ALT/AST elevation was observed in children with leukemia, 2 of whom had mild ALT elevation (62 and 65 U/L) at study entry. One DLT after completion of cycle 1 (150 mg/m²) was observed (grade 4 hyperglycemia).

Sorafenib’s most frequent non-DLTs were fatigue, rash, diarrhea, AST/ALT/bilirubin elevation, and predominantly mild myelosuppression (Table 3). No bony abnormalities were reported in 5 patients who underwent serial plain radiographs. Three patients required anti-hypertensive medication in reporting period 2 to 4 and were able to continue sorafenib at unchanged dosing.

Pharmacokinetics and pharmacodynamics

Twenty patients had pharmacokinetic sampling at day 1 and 17 patients at steady state.

There was substantial interpatient variability for day 1 and steady-state pharmacokinetic parameters (Table 4 and 5). Drug exposure increased only marginally when it was escalated from 150 to 200 mg/m² for day 1 pharmacokinetics (Table 4). The plasma concentration–time profile captured drug exposure incompletely (Fig. 1A). The terminal half-life appeared to be prolonged (≥24 hours) but could not be estimated. Sorafenib accumulated after multiple doses and steady state appeared to be achieved after 5 to 7 days (Table 4 and Fig. 1B) with the median accumulation ratio (day 27 trough/day 2 trough) ranging from 3.7 to 11.6. No statistically significant relationship between AUC_0–24h to DLTs or response was observed.

For steady-state pharmacokinetics, the accumulation ratio of sorafenib at the MTD [mean (SD) AUC_0–12h of 61.2 (18.2) steady state divided by mean (SD) calculated day 1 AUC_0–12h of 14.4 (8.9)] was 4.3. Steady-state sorafenib concentrations (AUC_0–12h/12) were 4.1 ± 2.1 μg/mL at the 150 mg/m² and 5.4 ± 1.8 μg/mL at 200 mg/m² dose levels, respectively. The median apparent sorafenib clearance at the MTD was 56.0 mL/min/m² (range, 19.2–72.8). The apparent sorafenib clearance increased with patient age (P = 0.048; Fig. 1C). No statistically significant relationship between AUC_0–12h and DLTs was observed.

In paired samples for plasma VEGF and endoglin concentrations (n = 26) and paired samples for CEC and CEPC number (n = 21), there was no change comparing baseline with end of cycle 1. A statistically significant decrease in soluble VEGFR2 (P ≤0.001) and increase in PlGF (P ≤ 0.001) was observed at steady state (Supplementary Fig. S2A and S2B). No correlations of the pharmacodynamic parameters to drug exposure or response were observed.

Response evaluation

The median number of sorafenib cycles for solid tumors was 2 (range, 1–22). There were no confirmed objective responses in patients with solid tumors but 1 patient (#24) with synovial sarcoma had more than 40% tumor shrinkage after cycle 1. This patient could not be considered a confirmed partial response, as disease progression was documented after cycle 2. Patients with leukemia received a median of 1 (range, 1–6) cycle. Two patients with AML and FLT3ITD achieved an M1 bone marrow (<5% blasts) after cycles 4 and 5, respectively, and were removed from sorafenib treatment to undergo hematopoietic stem cell transplantation.