Pentoxifylline, a methylxanthine that is used to treat veno-occlusive disease, can increase perfusion in undervascularized tissues. Addition of high concentrations, like caffeine, causes progression through radiation or drug induced G2 phase blocks, thereby limiting time for repair of DNA breaks and crosslinks. We have examined the potential of pentoxifylline to augment the effects of antitumor alkylating agents in vitro and in vivo. In MCF-7 human breast cancer cells in vitro, pentoxifylline (2 mM) present for 24 h was only slightly cytotoxic (approximately 10% cell kill at 2 mM), but when present prior to and during AA it increased the cytotoxicity of CDDP by 2 logs at 250 microM. With L-PAM in vitro, pentoxifylline was much less effective and only at a concentration of 250 microM L-PAM did 2 mM pentoxifylline increase cytotoxicity (approximately 0.3 logs). In the FSaIIC murine fibrosarcoma system, 100 mg/kg of pentoxifylline i.p. immediately prior to the alkylating agent or 50 mg/kg x 5 of pentoxifylline over 24 h with the alkylating agent given immediately after the third dose increased the tumor cell kill achieved by CDDP, carboplatin, cyclophosphamide, and thiotepa. The increase in tumor cell killing was modest (2.9-fold). Pentoxifylline in the multiple dose regimen (50 mg/kg x 5 over 24 h) was more effective than in the single dose (100 mg/kg) protocol. In the EMT6 mouse mammary adenocarcinoma, pentoxifylline (100 mg/kg daily x 5) improved the tumor growth delay produced by CDDP (3.3 mg/kg alternate days x 3), carboplatin (25 mg/kg daily x 5), cyclophosphamide (100 mg/kg alternate days x 3) and thiotepa 5 mg/kg (daily x 5). Only with cyclophosphamide, however, did the interaction appear to be large, as a 2.4-fold increase was observed.