TPR1, a novel rifampin derivative demonstrates efficacy alone and in combination with doxycycline against the NIAID Category A Priority Pathogen F. tularensis

Objectives Francisella tularensis is a highly virulent and contagious gram-negative intracellular bacterium that causes the disease tularemia in mammals and is classified as a Category A priority pathogen. The high infectivity, difficulty of obtaining a durable cure of disseminated disease and the emergence of drug resistance warrants investigation of new drugs that can be used alone and in combination with existing standard of care clinical drugs.

Methods We utilized a systematic analysis of antibacterial potency, extent of dissemination by analysis of bacterial burden in a secondary vital organ, and survival rates to assess the efficacy of a novel rifampicin derivative, TPR1. The efficacy of TPR1 was evaluated alone and in combination with the standard of care drug, doxycycline, against Type A F. tularensis Schu S4 using a lethal pulmonary model of infection in mice.

Results TPR1 has an MIC value range of 0.125-4 mg/L against reference laboratory strain SchuS4 and a panel of clinical strains. TPR1 alone reduced the bacterial burden in the lungs and spleen at 40 mg/kg and 80 mg/kg, and no antagonism was observed when co-administered with doxycycline. Dosing at 40 mg/kg doxycycline reduced the bacterial burden by 1 Log₁₀ CFU in the lungs and 4 Log₁₀ CFU spleen in comparison to untreated controls. Co-administration of TPR1 and doxycycline demonstrated efficacy upon treatment withdrawal after 4 days of treatment, and 100% survival.

Conclusions Significantly, TPR1 demonstrated efficacy when delivered alone and in combination with doxycycline, which provides compelling evidence of a superior treatment strategy that would normally rely on a single chemotherapeutic for efficacy. In addition, this work substantiates the use of rifampicin derivatives as a platform for the development of novel treatments to other bacterial agents in addition to tularemia.