Efficacy assessment of a novel endolysin PlyAZ3a(T) for the treatment of ceftriaxone-resistant pneumococcal meningitis in an infant rat model

BACKGROUND: Treatment failure in pneumococcal meningitis due to antibiotic resistance is an increasing clinical challenge and alternatives to antibiotics warrant investigation. Phage-derived endolysins efficiently kill gram-positive bacteria including multi-drug resistant strains, making them attractive therapeutic candidates. The current study assessed the therapeutic potential of the novel endolysin PlyAZ3a(T) in an infant rat model of ceftriaxone-resistant pneumococcal meningitis. METHODS: Efficacy of PlyAZ3a(T) was assessed in a randomized, blinded and controlled experimental study in infant Wistar rats. Meningitis was induced by intracisternal infection with 5 x 10(7) CFU/ml of a ceftriaxone-resistant clinical strain of S. pneumoniae, serotype 19A. Seventeen hours post infection (hpi), animals were randomized into 3 treatment groups and received either (i) placebo (phosphate buffered saline [PBS], n = 8), (ii) 50 mg/kg vancomycin (n = 10) or (iii) 400 mg/kg PlyAZ3a(T) (n = 8) via intraperitoneal injection. Treatments were repeated after 12 h. Survival at 42 hpi was the primary outcome; bacterial loads in cerebrospinal fluid (CSF) and blood were secondary outcomes. Additionally, pharmacokinetics of PlyAZ3a(T) in serum and CSF was assessed. RESULTS: PlyAZ3a(T) did not improve survival compared to PBS, while survival for vancomycin treated animals was 70% which is a significant improvement when compared to PBS or PlyAZ3a(T) (p<0.05 each). PlyAZ3a(T) was not able to control the infection, reflected by the inability to reduce bacterial loads in the CSF, whereas Vancomycin sterilized the CSF and within 25 h. Pharmacokinetic studies indicated that PlyAZ3a(T) did not cross the blood brain barrier (BBB). In support, PlyAZ3a(T) showed a peak concentration of 785 μg/ml in serum 2 h after intraperitoneal injection but could not be detected in CSF. CONCLUSION: In experimental pneumococcal meningitis, PlyAZ3a(T) failed to cure the infection due to an inability to reach the CSF. Optimization of the galenic formulation e.g. using liposomes might enable crossing of the BBB and improve treatment efficacy.