Locally delivered antistaphylococcal lysin exebacase or CF-296 is active in methicillin-resistant Staphylococcus aureus implant-associated osteomyelitis

Abstract

Introduction: Staphylococcus aureus is the most common cause of orthopedic infections and can be challenging to treat, especially in the presence of a foreign body. The antistaphylococcal lysins exebacase and CF-296 have rapid bactericidal activity, a low propensity for resistance development, and synergize with some antibiotics. Methods: Rabbit implant-associated osteomyelitis was induced by drilling into the medial tibia followed by locally delivering exebacase, CF-296, or lysin carrier. A titanium screw colonized with methicillin-resistant S. aureus (MRSA) IDRL-6169 was inserted. Intravenous daptomycin or saline was administered and continued daily for 4 d. On day 5, rabbits were euthanized, and the tibiae and implants were collected for culture. Results were reported as log ${}_{10}$ colony forming units (cfu) per gram of bone or log ${}_{10}$ cfu per implant, and comparisons among the six groups were performed using the Wilcoxon rank sum test. Results: Based on implant and bone cultures, all treatments resulted in significantly lower bacterial counts than those of controls ( $P\le 0.0025$ ). Exebacase alone or with daptomycin as well as CF-296 with daptomycin were more active than daptomycin alone ( $P\le 0.0098$ ) or CF-296 alone ( $P\le 0.0154$ ) based on implant cultures. CF-296 with daptomycin was more active than either CF-296 alone ( $P=0.0040$ ) or daptomycin alone ( $P=0.0098$ ) based on bone cultures. Conclusion: Local delivery of either exebacase or CF-296 offers a promising complement to conventional antibiotics in implant-associated infections.

Figure 1

Scanning electron microscopy of the MRSA-seeded implant.

Figure 2

Surgical procedure: an incision was made over the left medial tibia, and the smooth flat portion was exposed (a); using a micro drill, a hole was created through the cortical bone into the medullary cavity (b, c); a total of 2 mL of water (d) followed by 0.6 mL of lysin or lysin carrier (e) was injected; and a seeded implant was inserted (f), tightened with a screwdriver (g), and confirmed to be secure (h).

Figure 3

Medial and supine X-rays of the left rabbit tibia to show implant placement (arrow).

Figure 4

Implant time–kill study. Titanium screws grown overnight with MRSA IDRL-6169 were placed in 0.1 mL of lysin carrier, exebacase (10.64 mg mL ${}^{-\mathrm{1}}$ ), or CF-296 (10.16 mg mL ${}^{-\mathrm{1}}$ ). After 1, 2, 4, 8, 12, or 24 h, three screws each were processed for quantitative culture and results were reported as log ${}_{\mathrm{10}}$  cfu per implant.

Figure 5

Quantities of MRSA recovered from bones (a) and implants (b) on day 5. Dots represent values from individual animals; $n=\mathrm{10}$ per group; horizontal lines represent median values. Asterisks indicate significant reductions compared to * saline/carrier ( $P\le \mathrm{0.0025}$ ), ** daptomycin/carrier ( $P\le \mathrm{0.0098}$ ), or *** saline/CF-296 ( $P\le \mathrm{0.0154}$ ). (Note that slashes depict the treatment grouping, with the top being the systemic treatment given daily and the bottom being the local treatment administered at the time of surgery.)