Effects of hyperglycemia on lonidamine-induced acidification and de-energization of human melanoma xenografts treated with melphalan

Introduction: Melanoma is the most rapidly increasing form of human cancer in the United States, exhibiting a 4% increase in incidence per year since 1970 (1). In the US, melanoma ranked fifth in incidence among males and sixth among females in 2006 but was not among the top ten causes of cancer death for either gender (2). Surgical excision is the only proven therapy that leads to cure if the cancer is detected early. However, if recurrence occurs with metastasis, the prognosis is very poor since effective methods for treating the systemic disease are not available. Specifically, we seek to employ the natural tendency of melanomas and other tumors to convert glucose to lactate as a method for selective intracellular acidification of the tumor, which has been reported to potentiate tumor response to hyperthermia (3) as well as to chemotherapy with platinum (4) and N-mustard (5-9) alkylating agents. We performed this study to evaluate whether hyperglycemia induced selective intracellular acidification following lonidamine (LND) administration and substantial activity of melphalan (LPAM) by applying P-31 and hydrogen-1MR spectroscopy to monitor intra(pHi) and extracellular pH (pHe), tumor bioenergetics, and lactate levels respectively. We have reported before (9) that while LND alone had no significant effect on tumor growth delay, it substantially enhanced the activity of LPAM and did not substantially increase the toxicity of this antineoplastic agent. These findings point to the potential utility of nitrogen mustards and LND in the systemic treatment of disseminated melanoma. Material and Methods: Human melanoma xenografts development (n=10), pHi, pHe and bioenergetics (βNTP/Pi) estimation as well as lactate levels were performed as described elsewhere (9). LND (100 mg/kg; i.p.) was injected after 20 min following glucose infusion. A stock solution of D-glucose (2.5 M) diluted to 0.6 M and then delivered through a tail vein catheter with a variable rate using a syringe pump to maintain a blood concentration of 26 mM, as follows: (10ml/hr, 1min; 3ml/hr, 4min; 2.5ml/hr, 2min;