³¹P and ¹H MRS of DB-1 melanoma xenografts: Lonidamine selectively decreases tumor intracellular pH and energy status and sensitizes tumors to melphalan

In vivo ³¹P MRS demonstrates that human melanoma xenografts in immunosuppressed mice treated with lonidamine (LND, 100mg/kg intraperitoneally) exhibit a decrease in intracellular pH (pH_i) from 6.90±0.05 to 6.33±0.10 (p<0.001), a slight decrease in extracellular pH (pH_e) from 7.00±0.04 to 6.80±0.07 (p>0.05) and a monotonic decline in bioenergetics (nucleoside triphosphate/inorganic phosphate) of 66.8±5.7% (p<0.001) relative to the baseline level. Both bioenergetics and pH_i decreases were sustained for at least 3h following LND treatment. Liver exhibited a transient intracellular acidification by 0.2±0.1 pH units (p>0.05) at 20min post-LND, with no significant change in pH_e and a small transient decrease in bioenergetics (32.9±10.6%, p>0.05) at 40min post-LND. No changes in pH_i or adenosine triphosphate/inorganic phosphate were detected in the brain (pH_i, bioenergetics; p>0.1) or skeletal muscle (pH_i, pH_e, bioenergetics; p>0.1) for at least 120min post-LND. Steady-state tumor lactate monitored by ¹H MRS with a selective multiquantum pulse sequence with Hadamard localization increased approximately three-fold (p=0.009). Treatment with LND increased the systemic melanoma response to melphalan (LPAM; 7.5mg/kg intravenously), producing a growth delay of 19.9±2.0days (tumor doubling time, 6.15±0.31days; log₁₀ cell kill, 0.975±0.110; cell kill, 89.4±2.2%) compared with LND alone of 1.1±0.1days and LPAM alone of 4.0±0.0days. The study demonstrates that the effects of LND on tumor pH_i and bioenergetics may sensitize melanoma to pH-dependent therapeutics, such as chemotherapy with alkylating agents or hyperthermia.