Beta-cyclodextrin formulation of a disulfide-bond disrupting agent for improved systemic exposure†

Disulfide-bond disrupting agents (DDAs) are a class of cyclic thiosulfonates that have been shown to kill human epidermal growth factor receptor (HER) family-overexpressing breast cancer (BC) cells selectively and with no adverse side effects. Previous structure–activity relationships suggested a strong correlation between DDA lipophilicity and potency. In this study, we present the use of cyclodextrins (CDs) as molecular excipients to address the possible solubilizing drawback of increasingly lipophilic DDAs in oral administrations. The formulation of tcyDTDO, a potent second-generation DDA, with beta-cyclodextrin (BCD) and 2-hydroxypropyl-beta-cyclodextrin (HPB) was investigated. The choice of BCD as an optimal host over other CDs was guided by two in silico methods, namely: (1) host cavity volume estimations using a computational modeling approach and (2) binding energy (BE) calculations from simulations of different complexation geometries. A solid-state inclusion complex (IC) between tcyDTDO and BCD was prepared by kneading. Characterization by ATR-FTIR revealed positioning of tcyDTDO inside the cavity of BCD. Phase-solubility plots were constructed using NMR spectroscopy to measure the concentrations of host and guest in solution; a powerful technique that has yet to be exploited in the context of host–guest chemistry. The A_L-type plots obtained pointed to the formation of 1 : 1 complexes with both BCD and HPB. BCD formed a stronger complex with tcyDTDO (K_a of 4090 M⁻¹) although the solubility of tcyDTDO was enhanced by only 3-fold from an intrinsic solubility of 1.58 mM. Contrastingly, HPB displayed a lower affinity for tcyDTDO (K_a of 81 M⁻¹) but resulted in a remarkable 90-fold increase in solubility with tcyDTDO concentrations approaching 150 mM. Encapsulation of tcyDTDO in both cases did not hinder its anti-cancer activity as they retained cytotoxicity against MDA-MB-468 (EGFR+) BC cells in vitro. More striking was the superior pharmacokinetic profile and systemic exposure of tcyDTDO observed in male Sprague-Dawley rats when formulated with BCD as indicated by an area under the concentration vs. time curve (AUC_0–24) of 3150 ± 381 ng h mL⁻¹. This work suggests a correlation between K_a and in vivo pharmacokinetics of DDAs following their complexation with CDs and provides an ameliorated approach for their oral administration in future animal studies.