UCN-01 induces cytotoxicity toward human CLL cells through a p53-independent mechanism

Objectives UCN-01, a novel protein kinase C inhibitor, is currently being tested in phase I clinical trials after being noted to induce apoptosis in lymphoid cell lines. We sought to study the in vitro activity of UCN-01 against human chronic lymphocytic leukemia (CLL) cells and potential mechanisms of action for inducing this cytotoxicity. Methods Detailed in vitro studies were performed from tumor cells derived from patients with CLL cells following attainment of written informed consent. Results The 50% loss of viability (LC50) in mononuclear cells from CLL patients (n = 10) following exposure to UCN-01 for 4 days was 0.4 μM (95% CI ± 0.21; range 0.09–1.16). Loss of viability in human CLL cells correlated with early induction of apoptosis. Exposure of CLL cells to 0.4 and 5.0 μM of UCN-01 resulted in decreased expression of p53 protein. We therefore investigated the dependence of UCN-01 on intact p53 by exposing splenocytes from wild-type (p53+/+) and p53 null (p53−/−) mice, which demonstrated no preferential cytotoxicity when compared to the marked differential induced by F-Ara-A and radiation. Conclusion UCN-01 has significant in vitro activity against human CLL cells that appears to occur independent of p53 status. While demonstration of in vitro cytotoxicity does not establish in vivo efficacy, the findings described support the early introduction of UCN-01 into clinical trials for patients with B-CLL.