The idea of using deuterium to improve the properties of drugs is not entirely new.More than 50 years ago, in one of the earliest descriptions of the isotope′s effects on a drug′s pharmacodynamic properties, chemists from the Canada′s University of Ottawa, in collaboration with US scientists at what is now Bristol-Myers Squibb, reported that adding deuterium isotopes to a mol. that modulated the sympathetic nervous system limited its metabolism in cats (Science, 133, 102-104, 1961).A handful of companies pursued the strategy in the following decades, including the New Jersey drug giant Merck, which tried to use the process to reduce the toxicity of an antibiotic, but nothing ever panned out.So why the resurgence of interest now? "I asked myself that question a lot in the early period," says Pratik Shah, executive chairman of Auspex′s board of directors and a partner in the venture capital firm Thomas, McNerney & Partners. "What am I missing? Why hasn′t someone already done this?" One reason is that chem. techniques have improved significantly, notes Sheila DeWitt, chief executive of DeuteRx, which was spun out of Deuteria after its acquisition late last year. "Anal. methods have caught up to what is needed," she says.Between the various companies working in this space, a handful of deuterium-containing agents have gone into human trials.Auspex′s SD-809, a deuterated version of a drug called tetrabenzine, which is used for treating abnormal movement disorders, is the most advanced: in June, the company launched a 100-person, phase 3 study testing the SD-809 in people with Huntington′s disease.Meanwhile, Concert′s lead candidate-CTP-499, a first-in-class agent that aims to treat diabetic kidney disease by inhibiting multiple isoforms of the phosphodiesterase enzyme-is now undergoing phase 2 testing.Company scientists published pos. phase 1 safety data in Feb. (Clin. Pharmacol. Drug Dev.2, 53-56, 2013).CTP-499′s phase 2 trial is expected to wrap up late this year.Despite the clin. momentum, Vaz cautions that designing deuterium-containing drugs is far from straightforward. "Just sticking deuterium on a mol. won′t do it for you," he says.Vaz′s team at Pfizer has begun systematically looking at how to determine whether using deuterium will benefit a compound "The question is finding the right system that will translate to an in vivo effect," he says.Plus, even if the science can be worked out, success for deuterated drugs will ultimately require navigating a dizzying patent landscape that, in recent years, has undergone "a land-grab for deuterium-containing chem. entities," notes Shah.Dozens of patent applications for deuterated compounds have been approved, but since most are for improvements to already-existing agents, questions are increasingly being raised about whether the use of deuterium is novel and non-obvious.So far, companies have been able to argue that the isotope′s effects are not predictable.But as the space gets crowded, that could change. "Someday," predicts Vaz, "it′s probably going to go to court."