Siegel, E. L.; Rich, C.; Saravanan, S.; Pearson, P.; Xu, G.; Rich, S. M.

Borrelia burgdorferi\'s Inosine-5\'-Monophosphate dehydrogenase (IMPDH, GuaB) is a potential therapeutic target. GuaB is not needed for B. burgdorferi survival and replication in standard growth media, therefore our ability to test novel drugs against the whole organism is limited to tests using mammalian hosts. Modifications to standard culture conditions that can induce borreliastatic effects of GuaB inhibitors will enhance our ability to evaluate these inhibitors. This study aimed to evaluate basic modifications to standard growth media that can achieve this objective. Effects of GuaB inhibitors (mycophenolic acid, 6-Chloropurine) were compared in standard Barbour-Stoenner-Kelly-II (BSK-II) medium with BSK-II modified to 60% concentration rabbit serum, using B. burgdorferi 5A1. Effects were also evaluated with another BSK-II modification with the direct supplementation of hypoxanthine, adenine, and nicotinamide (75 M each). In standard BSK-II, neither mycophenolic acid nor 6-Chloropurine riboside affected B. burgdorferi growth. An ANOVA showed significant, dose-dependent interactions between culture condition and treatment considering modified growth conditions (F = 4.471, P = 0.001). Mycophenolic acid at 250 M reduced replication by 48 and 50% (P < 0.001 each). 6-Chloropurine riboside was more effective in both conditions at 250 M, reducing growth by 64 and 65% (P < 0.001 each). Effects of GuaB inhibitors were shown against whole organism Borrelia burgdorferi for the first time. Modification of BSK-II medium with physiologically relevant levels of mammalian serum supports replication and induces GuaB inhibitor-mediated suppression of replication. Effects of 6-Chloropurine riboside were demonstrated for the first time, indicating that B. burgdorferi can salvage and phosphorylate these analogue purine derivatives. These molecules can be manipulated to better inhibit replication. Optimisation will allow the assessment of novel Borrelia-specific IMPDH inhibitor molecules in culture for Lyme disease interventions.