Altered metabolism of cyclophosphamide given in a low-dose metronomic manner does not account for eventual resistance

Abstract

3185 Background: Low-dose metronomic chemotherapy (LDM), in particular regimens applying cyclophosphamide (CTX), has shown promising activity in preclinical and several phase I-II human studies comprising various tumor types. LDM is thought to act mainly via anti-angiogenic mechanisms (Nat Rev Cancer. 2004;4:423–36), and as such should be less prone to the rapid development of acquired resistance. However, in most instances tumors eventually relapse. Results obtained with PC-3 human prostate xenografts treated with daily oral LDM CTX suggest host-related resistance mechanisms that become functional after about 6 weeks of treatment. We hypothesized that modified CTX metabolism might contribute to this phenomenon. Methods: C57Bl/6J were treated with daily oral LDM CTX without breaks (Cancer Res. 2002;62:2731–5), and 4-hydroxy-CTX (4-OH-CTX) levels determined by mass spectrometry. Matrigel plug perfusion and tumor growth assays (PC-3 xenografts) were performed on mice ± pretreated for 8 weeks with LDM CTX. Cytochrome P450 3A4 activity was assessed in liver specimens obtained from mice under LDM CTX for 12 weeks. Results: 4-OH-CTX steady-state levels are in the 0.1 μg/ml range, which is above the IC50 obtained for endothelial cell proliferation in vitro (Cancer Res. 2002;62:6938–43). These levels are maintained over a period of at least 8 weeks. Pretreatement does not compromise the anti-angiogenic (Matrigel plug perfusion assay) or anti-tumor effects of LDM CTX. No induction of cytochrome P450 3A4 activity in liver microsomes was observed. Conclusions: Oral LDM CTX results in steady-state plasma levels of 4OH-CTX which have been shown previously to exert relevant anti-angiogenic activity in vitro. Our results suggest that altered metabolism of CTX does not account for the emergence of the resistance seen (pre)clinically. In contrast to maximum tolerated dose regimens, cytochrome P450 3A4 activity is not induced by LDM CTX. The latter is important since potential combination partners of LDM CTX and many drugs used in cancer care are metabolized by cytochrome P450 3A4. No significant financial relationships to disclose.