Other paradigms: growth rate constants and tumor burden determined using computed tomography data correlate strongly with the overall survival of patients with renal cell carcinoma

In solid tumors, where curative therapies still elude oncologists, novel paradigms are needed to assess the efficacy of new therapies and those already approved. We used radiologic measurements obtained in patients with metastatic renal cell carcinoma enrolled in a phase II study of the epothilone B analog, ixabepilone (Ixempra), to address this issue. Using a novel 2-phase mathematical equation, we used the radiologic measurements to estimate the concomitant rates of tumor regression and growth (regression and growth rate constants). Eighty-one patients were enrolled on the ixabepilone trial at the time of this analysis. Growth rate constants were determined using computed tomography measurements obtained exclusively while a patient was enrolled on study. The growth rate constants of renal cell carcinomas treated with ixabepilone were significantly reduced compared with those of tumors in patients who received placebo in a previous trial. Furthermore, a correlation with overall survival was found for both the growth rate constant and the initial tumor burden; and this correlation was even stronger when both the growth rate constant and the initial tumor burden were combined. The readily amenable mathematical model described herein has potential applications to many tumor types that can be assessed with imaging modalities. Because the growth rate constant seems to be a surrogate for survival, assessment could aid in the evaluation of relative efficacies of different therapies and perhaps in assessing the potential individual benefit of an experimental therapy.

The epothilones: how pharmacology relates to clinical utility

OBJECTIVE

To review the pharmacologic properties of a novel class of chemotherapeutic agents, the epothilones, and to summarize findings from recent clinical trials investigating the various epothilones in cancer therapy.

DATA SOURCES

Literature searches were conducted using MEDLINE, PubMed, and the abstract search engines for the American Society of Clinical Oncology and American Association for Cancer Research annual meetings (all searches through November 2008). Primary search terms included epothilone, BMS-247550, ixabepilone, EPO906, patupilone, sagopilone, and ZK-EPO.

STUDY SELECTION AND DATA EXTRACTION

Publications were given priority for inclusion if they discussed structural or pharmacologic properties of the epothilones as a class or if they included preclinical or clinical data for epothilones currently in clinical development.

DATA SYNTHESIS

The epothilones are a novel class of microtubule-stabilizing agents (MSAs). Epothilones are structurally and pharmacologically distinct from taxanes, but the exact ways in which the pharmacophores of the 2 classes differ has not been firmly established. A number of natural, semisynthetic, and fully synthetic epothilones are in various stages of clinical development. These agents differ from each other and from existing MSAs; these differences influence potency, stability, and solubility. Ixabepilone is currently approved to treat multidrug-resistant metastatic breast cancer and has demonstrated efficacy in earlier stages of breast cancer and in several other tumor types. Patupilone and sagopilone are currently under clinical investigation and have each shown promise in a number of treatment settings and tumor types. All 3 agents appear to be associated with manageable toxicities, but no class-wide toxicity profile exists for the epothilones and dose-limiting toxicities differ among the agents.

CONCLUSIONS

The epothilones have demonstrated significant potential for addressing the growing therapeutic challenge of taxane resistance, and the ever-increasing pool of information regarding structure-activity relationships of these MSAs will help to optimize microtubule-targeted chemotherapy.