Patient characteristics are summarized in . A total of fifty-seven patients were enrolled and 56 treated between July 2004 and February 2008. Colorectal cancer (CRC) was the most frequent cancer type (12 and 7 patients in the danusertib and danusertib + G-CSF groups, respectively). All patients had received prior systemic therapy, except for one patient with chondrosarcoma. More than one third had also received radiotherapy.
As shown in , the initial dose escalation over 7 DLs (45, 90, 180, 360, 500, 580, 650 mg/m2) was without G-CSF. Dose escalation through the 500 mg/m2 DL was well-tolerated. At the 650 mg/m2 DL (30% increment) 2 patients developed DLTs. Based on pharmacokinetics analysis (30% coefficient of variability in Cmax and AUC) we evaluated an intermediate DL of 580 mg/m2; there were 2 DLTs at this DL. Thus, 500 mg/m2 is the danusertib MTD without G-CSF. Based on DLTs of neutropenia we then amended the protocol to continue dose escalation with G-CSF primary prophylaxis. Sixteen patients were treated at 3 DLs with G-CSF (580, 750 and 1000 mg/m2). We did not identify a per protocol MTD with G-CSF but halted dose escalation and declared 750 mg/m2 the MTD based on 2 patients at the 1000 mg/m2 DL with elevated creatinine (azotemia) requiring a several day hospitalization for management with IV fluids. Forty patients treated with danusertib alone received a total of 159 cycles. A total of 66 cycles were administered to the 16 patients treated with danusertib with G-CSF (). The median number of cycles per patient was 3 with either treatment regimen (range 1–20 cycles for the danusertib only group and 1–12 cycles for the danusertib with G-CSF group). The median treatment duration was approximately 6 weeks in both groups (danusertib: 6.0 weeks [range 1.0–48.9]; danusertib with G-CSF: 6.1 weeks [range 0.9–24.0].
Dose Escalation Scheme, Treatment Duration, First Cycle DLT
The side effects of danusertib and the number of patients experiencing various grades of treatment-related toxicity (all cycles) are summarized by dose level and maximum CTC grade in . Toxicities were primarily hematological with febrile neutropenia as the Cycle 1 dose limiting toxicity in the group treated without G-CSF. Treatment emergent Grade 3–4 hematological toxicities occurred at all dose levels tested, with greater frequency at doses ≥360 mg/m2. In Cycle 1, median time to neutropenic nadir was 8 days with both regimens. Median time to recovery was shorter when danusertib was administered with G-CSF (3 days versus 7 days without G-CSF). The most frequent non-hematological adverse events in both groups were fatigue, anorexia, nausea, and vomiting, diarrhea and pyrexia. Most of these adverse events were reported at doses ≥360 mg/m2 (). Additional drug-related Grade 3–4 events were zoster with neutropenia (1 case); rigors with neutropenia (1 case); mucositis (2 cases), increased liver function tests and hypokalemia (1 case each).
Transient hypertensive episodes (defined as SBP > 150, or DBP > 100, or increase by 20 mmHg from pre-infusion values) were observed in 20% of pts receiving danusertib only (1 case each at 90, 180, and 500; 3 cases at 360 mg/m2 and 2 cases at 580 mg/m2) and in 19% of those receiving danusertib + G-CSF (1 case at each DL). Hypertensive episodes were observed mostly during Cycle 1, within 12 hours after the start of the infusion; were mild to moderate in severity, not dose-limiting and did not recur with repeated treatment. Hypertension was reported as a clinical AE (not related to study drug) in 1 patient treated at 580 mg/m2 with G-CSF. Six of the 12 patients experiencing hypertension were already on anti-hypertensive medication. We cannot exclude a role for danusertib in these hypertensive episodes as the drug does inhibit VEGFR in a submicromolar range in biochemical assays.
LVEF decreases to values below the lower limit of normal were detected by MUGA scan in 2 cases treated with danusertib alone at the 45 (−8%) and 360 mg/m2 DLs (−9%). LVEF reductions were asymptomatic and most recovered during or after treatment without specific intervention. Decreases of ≥15% were not observed.
Dose Limiting Toxicities
DLTs are summarized in . Based on the observed DLTs, the 500 mg/m2 DL is the MTD for the 24-h infusion without G-CSF. In the danusertib with G-CSF group, a first cycle DLT (febrile neutropenia) was reported in only 1 of 7 patients treated at 1000 mg/m2 DL.
Serious Adverse Events
Drug-related SAEs occurred in 12 patients (21%): 9 (22%) treated with danusertib without G-CSF (in 2, 3, and 4 cases at 500, 580, or 650 mg/m2, respectively) and in 3 patients (19%) receiving 1000 mg/m2 with G-CSF. In patients treated without G-CSF, SAEs were mainly hematological (febrile neutropenia, neutropenia, neutropenic infection, leucopenia, and anemia). Non-hematological drug-related SAEs included abnormal liver function tests, vomiting, rigors, pyrexia (1 case each). In patients receiving danusertib with G-CSF drug-related non-hematological SAEs included anorexia, nausea, renal failure, azotemia and pyrexia (1 case each).
Reasons for study discontinuation were disease progression (73% and 88% without and with G-CSF, respectively) or AEs (17% and 13% without and with G-CSF, respectively). One patient was never treated. The remaining patients completed treatment per protocol Dose reductions included 2 patients with grade 1 and 2 creatinine increases requiring medical treatment (IV hydration) at 1000 mg/m2 with G-CSF.
Danusertib pharmacokinetics parameters are summarized in . Mean ± SD plasma levels of the compound and its major metabolite at the MTD doses with and without G-CSF are presented in , respectively. The major metabolite observed in PK studies is the N-oxide of the N-methyl piperazine moiety; this has less than 1% of the activity of the parent compound in the proliferation assays and potency about 10 times lower than that of danusertib in a A2780 xenograft mouse model. After reaching maximal plasma concentration, danusertib showed a polyexponential decline. The pharmacokinetics of danusertib are characterized by high volume of distribution (average 13 L/kg) and low-to-moderate plasma clearance (range 0.33–0.46 L/hour/kg). Renal clearance accounted for a small proportion of plasma clearance (about 10%). No differences in half-life were seen across doses (average 18–26 hours). Plasma levels of the metabolite declined in parallel with those of the parent compound. The metabolite to parent AUC ratio was similar across doses and approximately equal to 1. Plasma concentrations of danusertib and its metabolite observed at selected time points in subsequent cycles were similar to those observed in Cycle 1. Systemic exposure to danusertib increased with dose (). Administration of G-CSF did not influence the pharmacokinetics of danusertib and its metabolite ().
Plasma PK Parameters (Mean± SD) of Danusertib during Cycle 1
Pre- and on-treatment skin biopsies were analyzed for changes in histone H3 phosphorylation (pH3) by Western Blot (WB) and immunohistochemistry (IHC) (Supplement B
, ). By WB most samples were not evaluable due to protein degradation; however, among evaluable samples a decrease in pH3 was observed in biopsies starting at 500 mg/m2
. This finding was confirmed by IHC evaluations. Although the number of pH3 positive cells detected was low, a trend towards down-modulation of pH3 was seen starting at 500 mg/m2
Objective tumor response was observed in one patient with refractory small cell lung cancer receiving 1000 mg/m2 danusertib + G-CSF (subsequently reduced to 750 mg/m2 for grade 2 creatinine elevation). Disease stabilization was observed in 18 of 42 evaluable pts (45.2%) (danusertib: 12 pts, 43%; danusertib + G-CSF: 7 pts, 50%). Prolonged disease stabilization (23.9–52.3 weeks) was observed in 4 patients (2 colorectal cancer, 1 ovarian cancer, 1 breast cancer) treated without G-CSF at 180, 500 (2 patients), and 650 mg/m2. In the patient with ovarian cancer tumor masses decreased 27% (RECIST) with a ≥30% decrease in CA125.