Well, technically that title should be PROTON, ELECTRON, and Hepatitis C, the first two words being the names of two recent studies of PSI-7977, a potential new drug for treating hepatitis C virus (HCV).
The Latest Findings
There’s a lot to talk about with PSI-7977—mainly in light of study resultspresented a few days ago at the 62nd Annual Meeting of the Association for the Study of Liver Diseases (AASLD) in San Francisco. So let’s get the elephant in the room out of the way before we go any further: I do not know what POSITRON and ELECTRON stand for. Nor do I know what FISSION, PROTON, and ATOMIC stand for—but more on that later. All I can tell you is that at some point in the history of drug development, pharmaceutical companies and/or clinical trial cooperative groups decided that acronyms were necessary or advantageous for some reason, paving the way for many a BLT, BOLERO and COMFORT for years to come.
PSI-7977 is kind of exciting. In the PROTON study, this drug, a nucleotide analog, was combined with the then-standard of care, pegylated interferon plus ribavirin. (Since PROTON was done, telaprevir and boceprevir were approved, changing the standard of care.) In PROTON, 96% of patients had a sustained virologic response (SVR), which is the measure of cure for HCV. Now, to balance this, is a wonderful moment of parsing the data: 96% is impressive, no doubt, but it has to be mentioned that the total number of patients in that study was 25, with 24 patients being actually evaluable. It was an early-phase study, so that small number of patients is not unusual, but most reports about the latest PSI-7977 results are highlighting that initial 96%, and it’s hard to find the actual N of the study. Here is a PDF of the full report of the PROTON study.
After PROTON delivered its encouraging results, Pharmasset, the maker of PSI-7977, launched ELECTRON, a phase II study in which a number of patients were given the experimental drug plus ribavirin. And that is the key: 10 of the enrollees received NO pegylated interferon. And guess what: the combination worked. All 10 of those HCV patients had an SVR.
Now, a couple of things to explain. First, these were patients with genotype 2 or 3 HCV. The reason why these genotypes were selected is because they tend to be highly responsive to interferon. Wait – so, why were those the people who were not given interferon? Well, the logic was that if PSI-7977 plus ribavirin didn’t work, those patients could be more easily rescued with a course of pegylated interferon + ribavirin than HCV patients with, say, genotype 1, the most difficult to treat variety of the disease. As it turned out, that rescue therapy wasn’t needed, but still, the logic is interesting when it comes to understanding drug trials.