Systemic rifampicin shows accretion to locally implanted hydroxyapatite particles in a rat abdominal muscle pouch model.

Introduction: biomaterials combined with antibiotics are routinely used for the management of bone infections. After eluting high concentrations of antibiotics during the first week, sub-inhibitory concentrations of antibiotics may lead to late repopulation of recalcitrant bacteria. Recent studies have shown that systemically given antibiotics like tetracycline and rifampicin (RIF) could seek and bind to locally implanted hydroxyapatite (HA). The aim of this in vivo study was to test if systemically administered rifampicin could replenish HA-based biomaterials with or without prior antibiotic loading to protect the material from late bacterial repopulation. Methods: in vivo accretion of systemically administered RIF to three different types of HA-based materials was tested. In group 1, nano (n)- and micro (m)-sized HA particles were used, while group 2 consisted of a calcium sulfate $/$ hydroxyapatite (CaS $/$ HA) biomaterial without preloaded antibiotics gentamycin (GEN) or vancomycin (VAN), and in group 3, the CaS $/$ HA material contained GEN (CaS $/$ HA $+$ GEN) or VAN (CaS $/$ HA $+$ VAN). The above materials were implanted in an abdominal muscle pouch model in rats, and at 7 d post-surgery, the animals were assigned to a control group (i.e., no systemic antibiotic) and a test group (i.e., animals receiving one single intraperitoneal injection of RIF each day (4 mg per rat) for 3 consecutive days). Twenty-four hours after the third injection, the animals were sacrificed and the implanted pellets were retrieved and tested against Staphylococcus aureus ATCC 25923 in an agar diffusion assay. After overnight incubation, the zone of inhibition (ZOI) around the pellets were measured. Results: in the control group, $2/6$ CaS $/$ HA $+$ GEN pellets had a ZOI, while all other harvested pellets had no ZOI. No pellets from animals in test group 1 had a ZOI. In test group 2, $10/10$ CaS $/$ HA pellets showed a ZOI. In test group 3, $5/6$ CaS $/$ HA $+$ GEN and $4/6$ CaS $/$ HA $+$ VAN pellets showed a ZOI. Conclusions: in this proof-of-concept study, we have shown that a locally implanted biphasic CaS $/$ HA carrier after 1 week can be loaded by systemic RIF administration and exert an antibacterial effect. Further in vivo infection models are necessary to validate our findings.