Diethylstilbestrol Nanosuspensions for Prostate Cancer: Subcutaneous Sustained Release Formulations and Pre-clinical PK/PD Evaluations

Prostate cancer is the most common type of cancer and the second leading cause of cancer related deaths in men in the United States. According to the National Cancer Institute (NCI), 238,590 new cases with deaths of 29,720 from prostate cancer have been estimated for 2013. Diethylstilbestrol (DES) was mainstay treatment for metastatic and castrate resistant prostate cancer for half a century. In the mid-1980s its use was drastically reduced because of its severe cardiovascular toxicity and thromboembolic complications such as deep vein thrombosis, pulmonary embolism and heart attack. At the same time, luteinizing hormone releasing hormone (LHRH) agonists were found to have similar efficacy profile to that of DES but with significantly lower toxicities. Thus LHRH agonists replaced DES. But, there has been a renewed interest in the treatment with DES since it has been shown to be efficacious in both androgen dependent prostate cancer and castrate resistant prostate cancer. Also, LHRH agonists with extensive use have started to show side effects. We are interested in applying pharmaceutical formulation approach to reduce the complications of DES while sustaining its systemic exposure. This would be a significant advancement for the DES therapy against prostate cancer. The side effects of DES have been associated with its hepatic exposure which is hypothesized to alter the coagulation cascade. The conventional oral DES because of its first pass effect led to a high hepatic exposure and significant toxicities. Thus, our aim was to develop a subcutaneous sustained release nanosuspension formulation of DES, which would circumvent the first pass effect of DES, potentially reducing the thromboembolic and cardiovascular complications and maintaining or improving the efficacy of DES by sustaining its release. Subcutaneous nanosuspension formulations of various particle sizes of 160 nm (NS-160), 300 nm (NS-300) and 500 nm (NS-500) were successfully formulated and extensively characterized. The in vitro release study demonstrated a sustained release of DES from the nanosuspensions as compared to that of the DES co-solvent preparation. Also, the in vitro release of DES was inversely dependent on the particle size of the nanosuspensions. The pharmacokinetic studies in the Sprague Dawley rats demonstrated that as compared to the oral DES suspension and subcutaneous co-solvent (solution), the DES nanosuspensions significantly sustained the release of DES and prolonged the circulation of DES. In addition, the DES nanosuspensions showed a longer elimination half life and slower clearance than the oral DES suspension. The DES nanosuspensions, with higher systemic exposure, showed a 20 times reduction of the hepatic exposure as compared to the oral DES suspension on the same dose basis. The subcutaneous DES nanosuspensions showed significantly lesser changes in the levels of the coagulation factors such as fibrinogen and anti-thrombin III and the rat blood clotting time as compared to those of oral DES suspension in the short term and long term toxicity studies. These effects could potentially decrease the cardiovascular toxicity and thromboembolic complications of DES. In the efficacy study in a prostate cancer tumor mouse model, the DES nanosuspension (NS-160), exhibited significant tumor suppression as compared to the oral DES suspension on the same dose basis. Also, the prostate specific antigen (PSA) levels elevated significantly with the progression of the tumors for treatment with oral DES but were maintained stable for the NS-160 treatment group over a period of 28 days. In conclusion, this study for the first time successfully demonstrated the viability of formulating a subcutaneous nanosuspension formulation of DES that achieved sustained release of DES, low hepatic exposures and potential reduction of oral DES complication with enhanced efficacy. We anticipate that the DES nanosuspension could be a lead candidate for further clinical trials on DES for its use against prostate cancer.