Equine platelet lysate shows in vitro antimicrobial activity after lyophilisation and storage

Background Platelet lysate (PL), an emerging biologic therapy, inhibits bacterial growth in vitro. Currently, PL is stored frozen at -80°C, which limits distribution and clinical access. Objectives To assess protein recovery and compare antimicrobial activity of PL before and immediately after lyophilisation, and after room-temperature storage. Study design Experimental in vitro study Methods Three pooled batches of PL from nine healthy equine donors were tested for cytokine content and in vitro efficacy in a time-kill assay as thawed PL, reconstituted freshly lyophilised PL (lyoPL), and reconstituted lyoPL stored for either 2 weeks or 6 months at room temperature (19–21°C). Time-kill assay focused on Escherichia coli and Staphylococcus haemolyticus. Experiments used a 50% (v/v) treatment in bacterial growth medium with phosphate-buffered saline (PBS) as control. Log₁₀ fold-changes in bacterial counts were compared between preparations using linear mixed-effects models and repeated-measures two-way ANOVA, with the pooled batch as the unit of replication (n = 3). Results LyoPL formulations contained similar total protein content to the fresh PL without necessitating addition of a cryobuffer. PDGF-BB was retained after lyophilisation, and immunoreactive CXCL4 (PF4) and CXCL7 concentrations did not differ between PL and lyoPL. Fresh lyoPL and thawed PL showed similar time-kill profiles. LyoPL retained antibacterial activity after 2 weeks and 6 months of storage. Against gram-positive S. haemolyticus, lyoPL and PL showed no significant differences irrespective of storage time and growth inhibition relative to PBS was preserved. Against gram-negative E. coli, PL and lyoPL stored for 2 weeks had a bactericidal effect whereas 6-month stored lyoPL was only bacteriostatic. Main limitations Three pooled batches, two bacterial isolates, one PL concentration, a 4-hour exposure period, and residual moisture not measured. Conclusions Lyophilised PL retained in vitro antimicrobial activity after reconstitution and room-temperature storage, with reduced activity against E. coli after 6 months.