Two in One: Size Characterization and Accelerated Short-Term Physical Stability of Dual-Drug Suspensions with Two Acidic Compounds (Indomethacin and Naproxen)

<b>Background/Objectives:</b> Co-delivering dual-drug systems have proven to be effective in, for example, anticancer therapy or HIV prophylaxis due to a higher target selectivity and therapeutic efficacy from compound synergism. However, various challenges regarding physical stability can arise during the formulation definition when multiple drug compounds are included in the same formulation. In this work, the focus was on aqueous suspensions, which could be applied as long-acting injectable formulations to release the drug compounds over weeks to months after administration. <b>Methods:</b> It was possible to gain insights into dual-drug nano- and microsuspensions containing two acidic compounds (indomethacin and naproxen) prepared by milling with dual centrifugation. Information regarding the physical stability of individual suspensions was subtracted and compared to dual-drug suspensions when prepared with the same milling conditions and stored at elevated temperatures of 40 °C. <b>Results:</b> Distinct particle size profiles after milling were obtained dependent on the stabilizer used in both individual and dual-drug suspensions. Most notably, the combination of indomethacin and naproxen in one formulation resulted in smaller sizes of drug particles compared to individual suspensions under the presence of some stabilizers. The obtained particle size profiles further indicated that at least one of the model compounds needed to be sufficiently stabilized from a stabilizer to obtain physically stable dual-drug suspensions over 28 days when stored at 40 °C. Similarly, the particle size distribution was dependent on the individual distribution of the suspensions, which showed a monomodal distribution could be achieved for dual-drug suspensions when at least one of the individual suspensions demonstrated a monomodal distribution in the presence of the stabilizer alone. Over a 28-day period, the smallest particle size was obtained in dual-drug suspensions stabilized with a combination of polysorbate 85 and poloxamer 338 compared to dual-drug suspensions stabilized with only a single stabilizer during preparation, indicating tendencies towards stabilization synergism from a combination of stabilizers as well as the model compounds. <b>Conclusion:</b> Overall, the study showed insights into the preparation and physical stability of dual-drug suspensions containing indomethacin and naproxen.