Antibacterial Activity of Clindamycin/BPO in Combination With Adapalene

Introduction: Acne guidelines recommend the addition of benzoyl peroxide (BPO) to antibiotics to reduce resistance in Cutibacterium acnes (C. acnes). Pairing the antibiotic/BPO combination with a retinoid, such as adapalene, may further increase treatment efficacy, although research on adapalene’s antibacterial activity is limited. To determine if adapalene improves antimicrobial activity, this in vitro study evaluated minimum inhibitory concentration (MIC) of clindamycin, adapalene, and BPO alone or in combination against both susceptible and resistant C. acnes isolates. Methods: Part 1a: C. acnes sensitivity to clindamycin, adapalene, or BPO was assessed via MIC values obtained by the broth microdilution method, with lower MIC indicating higher susceptibility. Part 1b: The effect (synergistic, additive, antagonistic, or indifferent [no interaction]) of combining adapalene with clindamycin or BPO on C. acnes inhibition was evaluated using a checkerboard assay, wherein two test compounds are combined in varying concentrations. Part 2: Susceptibility to single formulations of clindamycin, adapalene, and BPO, and fixed-dose combination formulations of an antibiotic (clindamycin or erythromycin) with BPO was determined by measuring antibacterial zone of inhibition using agar diffusion method, with larger diameter indicating increased bacterial inhibition. Results: Part 1a: Clindamycin demonstrated strain-dependent activity (MIC range: 512 and >64, respectively) against the 6 acne-associated strains tested. Part 1b: The combination of adapalene with clindamycin resulted in an additive effect for 3 and no interaction for 1 strain, whereas adapalene with BPO did not result in any interaction against the 4 acne-associated strains tested. Part 2: Activity of single formulations varied, with adapalene 0.1% having no activity (zone of inhibition: 0 cm) against any of the 8 acne-associated C. acnes strains tested. The fixed-dose combination formulations had generally similar activity against all strains tested (range: 0.9-5 cm). Conclusions: In these in vitro analyses, adapalene and BPO had no/low activity against C. acnes, whereas clindamycin alone or in combination formulations demonstrated strain-dependent activity. The relatively high MIC of BPO is not unexpected and aligns with previous studies in which the in vitro activity of BPO was low compared to its high in vivo antimi