Phase I clinical trial of continuous infusion cyclopentenyl cytosine

Abstract

Cyclopentenyl cytosine (CPE-C) is an investigational drug that is active against human solid tumor xenografts. The 5′-triphosphate of CPE-C inhibits CTP synthase, and depletes CTP and dCTP pools. We conducted a phase I clinical trial of CPE-C given as a 24-h continuous i. v. infusion every 3 weeks in 26 adults with solid tumors. The starting dose rate, 1 mg/m² per h, was selected on the basis of both preclinical studies and pharmacokinetic data from two patients obtained after a test dose of 24 mg/m² CPE-C as an i. v. bolus. Dose escalation was guided by clinical toxicity. A total of 87 cycles were given, and ten patients received four or more cycles. The mean CPE-C steady-state plasma levels (Cpss) increased linearly from 0.4 μM to 3.1 μM at dose levels ranging from 1 to 5.9 mg/m² per h (actual body weight); the mean total body clearance was 146±38 ml/min per m². CPE-C was eliminated by both renal excretion of intact drug and deamination to cyclopentenyl uracil in an apparent 2∶1 ratio. CTP synthase activity in intact bone marrow mononuclear cells was inhibited by 58% to 100% at 22 h compared to matched pretreatment samples at all CPE-C dose levels. When all data were combined, flux through CTP synthase was decreased by 89.6%±3.1% at 22h (mean ± SE,n=16), and remained inhibited by 67.6%±7.7% (n=10) for at least 24 h post-CPE-C infusion. Granulocyte and platelet toxicities were dose-dependent, and dose-limiting myelosuppression occurred during the initial cycle in two of three patients treated with 5.9 mg/m² per h. Four of 11 patients (4 of 20 cycles) who received 4.7 mg/m² per h CPE-C experienced hypotension 24–48 h after completion of the CPE-C infusion during their first (n=2), third (n=1) and sixth cycles (n=1), respectively. Two of these patients died with refractory hypotension despite aggressive hydration and cardiopulmonary resuscitation. One of 12 patients (28 total cycles) treated with 3.5 mg/m² per h CPE-C experienced orthostatic hypotension during cycle 1, and this patient had a second episode of orthostatic hypotension at a lower dose (3.0 mg/m²per h). Hypotension was not seen in patients receiving ≤2.5 mg/m² per h CPE-C. The occurrence of hypotension did not directly correlate with either CPE-C Cpss, CPE-U plasma levels, pretreatment cytidine plasma levels, baseline CTP synthase activity, or with the degree of enzyme inhibition during treatment. While the hypotension appeared to be dose-related, its unpredictable occurrence and the uncertainty concerning the mechanism preclude a recommendation of a tolerable dose for future studies.