CLINICAL PHARMACOLOGY
Mechanism of Action:
Clofarabine is sequentially metabolized intracellularly to the 5’-monophosphate metabolite by deoxycytidine kinase and mono- and di-phospho-kinases to the active 5’-triphosphate metabolite. Clofarabine has high affinity for the activating phosphorylating enzyme, deoxycytidine kinase, equal to or greater than that of the natural substrate, deoxycytidine. Clofarabine inhibits DNA synthesis by decreasing cellular deoxynucleotide triphosphate pools through an inhibitory action on ribonucleotide reductase, and by terminating DNA chain elongation and inhibiting repair through incorporation into the DNA chain by competitive inhibition of DNA polymerases. The affinity of clofarabine triphosphate for these enzymes is similar to or greater than that of deoxyadenosine triphosphate. In preclinical models, clofarabine has demonstrated the ability to inhibit DNA repair by incorporation into the DNA chain during the repair process. Clofarabine 5’-triphosphate also disrupts the integrity of mitochondrial membrane, leading to the release of the pro-apoptotic mitochondrial proteins, cytochrome C and apoptosis‑inducing factor, leading to programmed cell death.
Clofarabine is cytotoxic to rapidly proliferating and quiescent cancer cell types in vitro .
Human Pharmacokinetics
The population pharmacokinetics of Clolar® were studied in 40 pediatric patients aged 2 to 19 years (21 males/19 females) with relapsed or refractory acute lymphoblastic leukemia (ALL) or acute myelogenous leukemia (AML). At the given 52 mg/m2 dose, similar concentrations were obtained over a wide range of body surface areas (BSAs). Clofarabine was 47% bound to plasma proteins, predominantly to albumin. Based on non-compartmental analysis, systemic clearance and volume of distribution at steady-state were estimated to be 28.8 L/h/m2 and 172 L/m2, respectively. The terminal half-life was estimated to be 5.2 hours. No apparent difference in pharmacokinetics was observed between patients with ALL and AML or between males and females.
No relationship between clofarabine or clofarabine triphosphate exposure and toxicity or response was found in this population.
Based on 24-hour urine collections in the pediatric studies, 49-60% of the dose is excreted in the urine unchanged. In vitro studies using isolated human hepatocytes indicate very limited metabolism (0.2%), therefore the pathways of non-renal elimination remain unknown.
Although no clinical drug-drug interaction studies have been conducted to date, on the basis of the in vitro studies, cytochrome p450 inhibitors and inducers are unlikely to affect the metabolism of clofarabine. The effect of clofarabine on the metabolism of cytochrome p450 substrates has not been studied. The pharmacokinetics of clofarabine have not been evaluated in patients with renal or hepatic dysfunction.
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CLINICAL STUDIES
Sixty-six (66) pediatric ALL patients were exposed to Clolar®. Fifty-eight (58) of the patients received the recommended pediatric dose of Clolar® 52 mg/m2 daily x 5 as an intravenous (IV) infusion.
The safety and efficacy of Clolar® were evaluated in pediatric patients with refractory or relapsed hematologic malignancies in an open-label, dose-escalation, noncomparative study. The starting dose of Clolar® was 11.25 mg/m2/day IV infusion daily x 5 and escalated to 70 mg/m2/day IV infusion daily x 5. This dosing schedule was repeated every 2 to 6 weeks depending on toxicity and response. Nine of 17 ALL patients were treated with Clolar® 52 mg/m2 daily x 5. In the 17 ALL patients there were 2 complete remissions (12.5%) and 2 partial remissions (12.5%) at varying doses. Dose-limiting toxicities (DLTs) in this study were reversible hyperbilirubinemia and elevated transaminase levels and skin rash, experienced at 70 mg/m2. As a result of this study, the recommended dose for subsequent study in pediatric patients was determined to be 52 mg/m2/day for 5 days.
Single Arm Study in Pediatric ALL
A single arm study was conducted in relapsed/refractory pediatric patients with ALL at a single dose. All patients had disease that had relapsed after and/or was refractory to two or more prior therapies. Most patients, 46/49 (93.8%), had received 2 to 4 prior regimens and 15/49 (30.6%) of the patients had undergone at least 1 prior transplant. The median age of the treated patients was 12 years. There were more males, 29/49 (59.2%), than females, 20/49 (40.8%). Most of the patients were either Caucasian (n=20, 40.8%) or Hispanic (n=20, 40.8%), with 12.2% African-American (n=6), and 6.1% Other race (n=3). All patients received a dose of 52 mg/m2 daily x 5 IV infusion. There was no dose modification during the remission induction phase of treatment (maximum of 2 cycles). Doses could be modified (reduced/delayed) during the post-induction phase. There was no dose escalation. The planned study endpoint was the rate of Complete Remission (CR), defined as no evidence of circulating blasts or extramedullary disease, an M1 bone marrow (<5% blasts), and recovery of peripheral counts [platelets > 100 x 109/L and absolute neutrophil count (ANC) > 1.0 x 109/L] and Complete Remission in the Absence of Total Platelet Recovery (CRp), defined as meeting all criteria for CR except for recovery of platelet counts to > 100 x 109/L. Partial Response (PR) was also determined, defined as complete disappearance of circulating blasts, an M2 bone marrow (≥5% and ≤25% blasts), and appearance of normal progenitor cells or an M1 marrow that did not qualify for CR or CRp. Transplantation rate was not a study endpoint.
Response rates for these studies were determined by an unblinded Independent Response Review Panel (IRRP).
Table 1 summarizes results for the pediatric ALL study. Responses were seen in both pre-B and T-cell immunophenotypes of ALL. The median cumulative dose was 540 mg (range 29-1905 mg) in 1 (42.9%), 2 (38.8%) or 3 or more (18.4%) cycles.
Table 1: Results in Pediatric ALL Study | n=49 |
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| Responses | n | % | 95% CI |
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CR
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6
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12.2
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4.6 to 24.8
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CRp
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4
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8.2
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2.3 to 19.6
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PR
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5
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10.2
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3.4 to 22.2
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Of the 15 responding pediatric ALL patients, 6 had post-clofarabine bone marrow transplantation, so that duration of response could not be determined. In the 9 responding patients who were not transplanted, the response durations for CR were 43, 50, 82, 93+, and 160+ days; for CRp the response duration was 32 days; and for PR the response durations were 7, 16, and 21 days.
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