GlaxoSmithKline halted development of the antibiotic drug candidate GSK2251052, designed to treat complicated urinary tract and Gram-negative bacterial infections, and will return rights for the compound to its collaboration partner Anacor Pharmaceuticals.

Anacor said today GSK made the decision after identifying microbiological resistance in “a small number of” patients in a Phase IIb trial for complicated urinary tract infections (cUTI), followed by additional preclinical research.

GSK paused patient recruitment for the trial and all three others involving GSK2251052 in February, citing the resistance. At the time, GSK and Anacor were also evaluating the compound in a Phase IIb study in complicated intra-abdominal infections, and two Phase I studies in healthy volunteers. While insisting the resistance was unrelated to the safety of the compound, the pharma giant was concerned enough about its potential to negatively impact efficacy to voluntarily suspend enrollment in all four studies.

Anacor licensed the drug candidate, once called AN3365, exclusively to GSK in July 2010, the first compound the companies agreed to co-develop under an R&D collaboration launched in 2007. GSK assumed responsibility for development of the compound and any resulting commercialization. In return, Anacor received an option exercise fee of $15 million, and was eligible for further development and commercialization milestone payments and royalties on any future product sales.

In September 2011, the U.S. Biomedical Advanced Research and Development Authority (BARDA) agreed to award GSK $38.5 million over two years toward development of GSK2251052 against both hospital Gram negative and biothreat pathogens—specifically, by focusing on the target of bacterial enzyme leucyl tRNA synthetase. The award included options by BARDA that would have raised the value of the award to $94.5 million.

In connection with the BARDA agreement, Anacor last year received $5 million upfront from GSK, plus eligibility for royalties on future sales of resulting products, and the possibility of additional milestone payments, bonus payments, and research funding, the company disclosed in its 10-Q filing of second-quarter results with the U.S. Securities and Exchange Commission.

“Such innovative public-private collaborations provide an additional stimulus for antibacterial research and development,” David Payne, GSK’s head of antibacterial drug discovery, said in a statement at the time.

At the same time, GSK agreed to fund Anacor’s tuberculosis research activities through to selection of a drug candidate against the disease. That accord gave GSK the option to license such compounds, then take responsibility for all further development and commercialization of such compounds. In return, Anacor would be eligible for bonus payments tied to development and regulatory events, as well as to undisclosed sales levels for a TB compound. Anacor would also be eligible for royalties on sales of resulting products.

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