Potentiation of anti-cancer drug activity at low intratumoral pH induced by the mitochondrial inhibitor m-iodobenzylguanidine (MIBG) and its analoguebenzylguanidine (BG)

Tumour-selective acidification is of potential interest for enhanced therap eutic gain of pH sensitive drugs. In this study, we investigated the feasib ility of a tumour-selective reduction of the extracellular and intracellula r pH and their effect on the tumour response of selected anti-cancer drugs. In an in vitro L1210 leukaemic cell model, we confirmed enhanced cytotoxic ity of chlorambucil at low extracellular pH conditions. In contrast, the al kylating drugs melphalan and cisplatin, and bioreductive agents mitomycin C and its derivative EO9, required low intracellular pH conditions for enhan ced activation. Furthermore, a strong and pH-independent synergism was obse rved between the pH-equilibrating drug nigericin and melphalan, of which th e mechanism is unclear. In radiation-induced fibrosarcoma (RIF-1) tumour-be aring mice, the extracellular pH was reduced by the mitochondrial inhibitor m-iodobenzylguanidine (MIBG) or its analogue benzylguanidine (BG) plus glu cose. To simultaneously reduce the intracellular pH, MIBG plus glucose were combined with the ionophore nigericin or the Na+/H+ exchanger inhibitor am iloride and the Na+-dependent HCO3-/Cl- exchanger inhibitor 4,4'-diisothioc yanostilbene-2,2'-disulphonic acid (DIDS). Biochemical studies confirmed an effective reduction of the extracellular pH to approximately 6.2, and anti -tumour responses to the interventions indicated a simultaneous reduction o f the intracellular pH below 6.6 for at least 3 h. Combined reduction of ex tra- and intracellular tumour pH with melphalan increased the tumour regrow th time to 200% of the pretreatment volume from 5.7 +/- 0.6 days for melpha lan alone to 8.1 +/- 0.7 days with pH manipulation (P < 0.05). Mitomycin C related tumour growth delay was enhanced by the combined interventions from 3.8 +/- 0.5 to 5.2 +/- 0.5 days (P < 0.05), but only in tumours of relativ ely large sizes. The interventions were non-toxic alone or in combination w ith the anti-cancer drugs and did not affect melphalan biodistribution. In conclusion, we have developed non-toxic intetventions for sustained and sel ective reduction of extra- and intracellular tumour pH which potentiated th e tumour responses to selected anti-cancer drugs.