Updated Results of a Phase I Study of Flavopiridol in Acute Leukemias Using a Novel, Pharmacokinetically Derived Schedule: Clinical Activity Including Hyperacute Tumor Lysis Syndrome (TLS), Pharmacokinetics (PK), and Pharmacodynamics (PD).

Flavopiridol is a cyclin-dependent kinase inhibitor that induces apoptosis in acute leukemia cell lines. In earlier studies with flavopiridol in several malignancies, plasma concentrations of flavopiridol were not reached or maintained at sufficient levels to induce apoptosis due to unexpectedly high levels of bound drug in human serum relative to fetal calf serum used in preclinical studies. PK modeling in chronic lymphocytic leukemia (CLL) cells cultured in human plasma in vitro indicated that administering flavopiridol by 30 minute intravenous (IV) bolus followed by 4 hour continuous IV infusion (CIVI) would achieve sustained in vivo plasma drug concentration and time exposure similar to that necessary to induce apoptosis. We designed a phase I dose escalation trial in acute leukemias of single agent flavopiridol given as a 30 minute bolus followed by a 4 hour CIVI on days 1–3 with the ability to repeat cycles every 21 days; 16 patients (pts) have been enrolled to date. Dose escalation was as follows (bolus dose/4 hr CIVI dose in mg/m²): 20/30 (n=3), 30/35 (n=7), 30/50 (n=3), and 40/60 (n=3). Based on prior experience with flavopiridol at our institution in CLL, aggressive measures for the prevention and management of hyperacute tumor lysis syndrome (TLS) were employed. Pts had relapsed/refractory AML (N=12) and ALL (N=4), and were 25–78 yrs old (median age 64 yrs). Average plasma levels were 1.0–2.5 μM at the first three dose levels during the infusion (N=13) and declined with terminal half-lives comparable to previously reported 72 hr and more recent 4.5 hr infusions. Clinically significant TLS occurred in 2/16 pts with chemical evidence of lysis in 4 additional pts. A dose-limiting toxicity (renal failure) occurred at dose level 4 (40 mg/m² bolus/60 mg/m² CIVI), and the level is currently being expanded. Treatment was otherwise well tolerated. Downregulation of Mcl-1 protein by standard immunoblotting at 4 and/or 24 hrs was demonstrated in blood and/or bone marrow cells of 6/10 patients Anti-leukemic activity including transient reductions in WBCs/circulating blasts (n=7), bone marrow blasts (n=2), and platelet transfusion independence (n=1) was observed. Two received a second course of therapy, but no pt experienced an objective response by standard criteria. The current dose level exceeds that previously given in ongoing CLL studies; dose escalation to identify the maximum tolerated dose using this pharmacokinetically derived schedule in acute leukemia continues. Given the activity of this drug as a single agent, combination studies with conventional chemotherapy or other novel agents in acute leukemias should be considered.