Schedule-dependent interactions between perifosine and radiotherapy in prostate cancer

Abstract

Objective Perifosine was developed as a membrane-targeted alkylphospholipid that inhibits the PI3K/AKT pathway and has been suggested as a favorable candidate for combined use of radiotherapy. To better define the optimal schedule for this combination, we investigated schedule-dependent cytotoxic effects of perifosine and radiotherapy against prostate carcinoma cell lines in vitro. Methods Human prostate cancer cell line CWR22RV1 and LNCaP were incubated with perifosine (5, 20, 40 μM) and treated with radiation (2, 4, 6, 8 Gy). Three different schedules were used: simultaneous treatment (R+P), perifosine followed by radiotherapy (P→R), and vice versa (R→P). Cell growth inhibition was determined by 3-(4,5-dimethylthiazol-2-yl)5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay. The effects of drug combinations at the concentration producing 80 % cell growth inhibition were analyzed by the isobologram method (Steel and Peckham). Colony formation assay was also conducted to confirm the result of MTS assay. Western blot analysis was used to explore the molecular mechanism of action for the combination of perifosine and irradiation. Results The R→P schedule showed the most significant cell growth inhibition when compared to other schedules (p<0.01). This schedule produced an additive effect for CWR22RV1 and a supra-additive effect for LNCaP. The reversesequenceproducedaprotectiveeffectforCWR22RV1 and an additive/sub-additive effect for LNCaP. R+P schedule showed intermediate effect between R→ Pa nd P→ Rs chedules. Western blot analysis showed that phosphorylation of AKT (indicative of anti-apoptosis activity) was inhibited by this combination therapy except for P→ Rs chedule. Conclusion Our new findings suggest that R→ Pw ould be the optimal schedule for this combination treatment strategy. The finding is also important as it explains the negative outcomes of several previously reported trials combining radiotherapy and perifosine in the treatment of various tumors. Applications of this schedule-dependent approach might be beneficial for the treatment of prostate cancer and other tumors.