Assessing Pharmaceutical Equivalence of Generic Antibiotics Using in vitro Antimicrobial Susceptibility of Some Hospital Strains in Rwanda

The study aimed at evaluating the pharmaceutical equivalence of generic commercial products containing same common antibiotics by testing in vitro the antimicrobial susceptibility/resistance of common bacterial isolates from patients. In total 35 different generic preparations and brands corresponding to seven antibiotics- Ampicillin, Amoxicillin, Amoxicillin/clavulanic acid, Cotrimoxazole, Norfloxacin, and Erythromycin- were compared by a disc diffusion method against three pathogenic strains- Salmonella typhi, Shigella flexneri and Staphylococcus aureus- isolated from patients. Some brands were presumptively regarded as good quality medicines to serve as gold standards instead of using international references. The pharmaceutical quality of the preparations was checked by visual inspection and identification of active ingredients according to referral pharmacopeias. All products satisfied visual inspection and identification tests. However they exhibited differences in their antimicrobial profiles and potency. Two generic preparations containing amoxicillin/clavulanic acid were out of specifications (<90%) as compared with Augmentin® gold standard. Comparing the susceptibility of bacteria by the diameter (d) of inhibition zone in mm, Salmonella typhi was susceptible to Norfloxacin (d=23.2), low to Augmentin (d=11.5), and resistant to Ampicillin, Amoxicillin, Cotrimoxazole and Erythromycin (d=0). Shigella flexneri was susceptible to all antibiotics (d=31.6 – 42.8) except Erythromycin (d=0) which exhibited the lowest spectrum of activity. Staphylococcus aureus was susceptible to all antibiotics with different potencies (d=20 for Amoxicillin – d=42.6 for Norfloxacin). These findings showed the possibility of using a simple in vitro antimicrobial susceptibility testing to compare the equivalence of marketing antibiotic products in quality and efficacy. The result also could help clinicians choosing the most appropriate antibiotic in treatment.