There has long been a love-hate relationship with ddI (didanosine, Videx^®) in the HIV community. The drug has repeatedly demonstrated its potency and laboratory studies (with some confirmation from human studies) show that resistance to ddI develops very slowly. However, the current formulation of ddI contains an antacid buffer to help the drug survive its passage through the stomach (ddI breaks down quickly in the presence of stomach acid). Similarly, ddI must also be taken on an empty stomach (at least a half hour before a meal), when stomach acid levels are low. The antacid buffer further complicates the use of ddI in combination with some other drugs. For instance, a combination which includes ddI and indinavir (Crixivan^®) is extremely difficult to take because both drugs need to be taken on an empty stomach but the buffer in ddI, which helps that drug, will reduce the amount of indinavir found in blood if they are taken at the same time. As a result the two drugs have to be taken at least an hour apart, which can make scheduling the drugs a challenge. In addition, the antacids are also associated with diarrhea and nausea, two side effects commonly reported by people taking ddI.

There is finally some good news on this subject. Bristol-Myers Squibb has developed a new formulation of ddI designed to solve these problems. It is currently being tested in studies. This new formulation eliminates the antacid buffer, simplifying the use of ddI in combination therapy. This should also reduce some of the diarrhea and nausea associated with the current formulation. People will be able to take the new ddI at the same time as indinavir, although the restrictions on taking both drugs on an empty stomach will still apply. Unfortunately, the Food and Drug Administration has requested that long-term studies be conducted to determine if the new formulation works as well as the old one. As a result, the new formulation ddI will not be available, outside of clinical trials, until the latter half of next year.

On another front, results from two studies suggest that ddI can be safely taken once a day rather than the currently recommended twice daily dosing. Both studies found that 400mg of ddI, taken once a day, was as effective in decreasing HIV levels and increasing CD4+ cell counts as 200mg of ddI taken twice a day. Participants in these studies also received d4T (stavudine, Zerit^®) dosed twice a day. The incidence of side effects was similar between the two groups with elevations in liver enzymes being the most common. The once a day regimen makes ddI a little more user-friendly.

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