Development of a lyophilised RH1 formulation: a novel DT diaphorase activated alkylating agent

RH1 is a novel aziridinylbenzoquinone alkylating agent, which is activated in tumour cells by DT diaphorase. In common with previous aziridinylbenzoquinones, RH1 exhibits limited aqueous stability and solubility. The aim of this study was to examine the pharmaceutical properties of RH1 with a view to preparing a suitable formulation for clinical trial. Stability in a neutral phosphate-buffered solution was poor with a degradation half-life of 50 h at 55 degrees C, indicating that lyophilisation was preferable. The reaction kinetics indicated a similarity with previous studies for base-catalysed degradation of aziridinylbenzoquinones. Intrinsic aqueous solubility at 0.5 mg mL(-1) may be increased in solvent systems or by the use of polymers such as polyvinylpyrrolidone (PVP) or complexing agents like hydroxypropyl-beta-cyclodextrin (HPBCD). In the latter case this increased solubility by an order of magnitude to around 5 mg mL(-1). Four potential formulations based on lyophilisation of RH1 (1 mg mL(-1)) from buffered solution (pH 7, 0.01 M NaH2PO4) containing either 50 mg mL(-1) mannitol, 40 mg mL(-1) dextran, 20 mg mL(-1) PVP or 50 mg mL(-1) HPBCD were prepared and examined for stability characteristics. All formulations exhibited a temperature-dependent degradation. The mannitol and dextran formulations had limited stability and degraded rapidly at all temperatures. The PVP and HPBCD formulations degraded at elevated temperatures but remained stable for up to twelve months at 4 degrees C. Examination of the degradation kinetics in the latter systems demonstrated similarity to the solution degradation mechanism, while in the former alternative degradation pathways appeared to be occurring. The chemical stability of RH1 in lyophilised formulations is dependent upon the excipient employed and storage temperature. Either the PVP or HPBCD formulation would be suitable clinical trial formulations of RH1. The results indicate that the choice of lyophilisation excipient for aziridinylbenzoquinones cannot be based on previous literature studies of related agents.