Objective: Thermal therapy can increase tumor uptake of chemotherapy drugs via a number of mechanisms, contributing to greater anti-tumor efficacy. We have previously reported a significantly enhanced therapeutic effect of long-duration (6h), whole body, fever-range (40^oC) thermal therapy combined with oxaliplatin chemotherapy in an in vivo breast tumor model. The efficacy of platinum chemotherapy agents (cisplatin, carboplatin, lobaplatin, oxaliplatin) has been correlated with platinum-DNA adduct formation. We hypothesized that cellular uptake of oxaliplatin and adduct formation is increased by fever-range hyperthermia and may explain the observed acute thermal enhancement of oxaliplatin efficacy. Possible mechanisms of thermal enhancement of oxaliplatin efficacy were investigated in vitro in MTLn3 mammary adenocarcinoma breast cancer cells by examining cell kill, platinum uptake, and platinum-DNA adduct formation.

Methods: Tumor cell kill by oxaliplatin was measured by the MTT assay. Platinum concentration in tumor cell lysates, and platinum bound to DNA extracted from tumor cells, was measured by Flameless Atomic Absorption Spectrometry (FAAS) following incubation of MTLn3 cells with oxaliplatin for 6h at 37^oC and at 40^oC.

Results: Fever-range heat decreased the IC-50 concentration of oxaliplatin by 50% from 0.11 μg/ml to 0.05 μg/ml. Cellular platinum concentration was increased 34% from 307 to 412 ng/mg protein in heated cells treated with oxaliplatin compared to oxaliplatin alone at 37^oC (p=0.017). Platinum bound to DNA was increased by 36% from 60 to 82 ng/mg DNA (p=0.079) in heated cells. The ratio of the concentration of platinum-DNA adducts to total intracellular platinum concentration was the same for oxaliplatin alone at 37^oC and oxaliplatin with 40^oC heat, namely 0.197 and 0.199, respectively.

Conclusions: Long duration, fever-range hyperthermia increases cellular uptake of oxaliplatin, which in turn increases the level of DNA adducts. This is consistent with the observed thermal enhancement of cell kill and tumor growth delay in vivo. The constant relationship between cytosolic platinum levels and DNA adducts formed indicates that thermal perturbation of the cell membrane may be the primary cause of the increase in DNA adducts, rather than increased reaction kinetics between platinum and DNA. Thermal therapy promises to be an effective adjunct to oxaliplatin chemotherapy.