Long-Term Stability of a Fixed-Dose Doxazosin and Finasteride Combination Formulation

Lower urinary tract symptoms (LUTS) secondary to benign prostatic hyperplasia (BPH) are commonly treated with a1-adrenoreceptor antagonists, 5a-reductase inhibitors, or a combination of both. The development of a fixed-dose doxazosin (DOX) and finasteride (FIN) formulation in a single dosage form is expected to improve efficacy, lower cost, and improve adherence to the treatment of male LUTS/BPH.

The study aims to determine the stability of a fixed-dose combination formulation containing doxazosin (1 mg) and finasteride (5 mg).

We conducted long-term stability studies of a fixed-dose DOX-FIN combination capsule. A stability-indicating HPLC method was developed and used to determine DOX and FIN in combination capsules simultaneously. DOX-FIN capsules were stored at 4°C, 25°C, and 40°C for 12 months. Samples were analyzed periodically using the HPLC method.

The HPLC analysis showed that both DOX and FIN amounts remained above 90% of the initial values after 1 year when the capsules were stored at 4°C and 25°C. An elevated temperature (40°C) is not a recommended storage condition for the compounded capsules. Even at 40 °C, doxazosin stability remains above 90% up to 30 days, and finasteride remains above 90% up to 90 days.

Long-term stability studies indicated that the combination of doxazosin and finasteride remained stable, with 90% of the drug still present in the formulation after one year for capsules stored at 4°C and 25°C.