- The Prilosec-Nexium scenario: Many drugs are racemic mixtures of two isomers, mirror-image molecules, of which only one is biologically active. The "new" drug is simply the active isomer of the old drug. (For some reason, when a drug is just being released to the market, it never seems worth the company's while to go to the trouble of separating out the active isomer. This only seems to suddenly appear to be an advantage when the parent drug is about to go off-patent.)
- The Claritin-Clarinex scenario: The "new" drug is the active metabolite of the old drug--what the body turns the old drug into after it is ingested.
If the company proceeds true to form, the Janssen reps that last week were telling all my fellow physicians that Risperdal was the greatest drug ever invented for psychosis, and for calming down the disturbed, demented elderly patient, will now be telling the docs that Risperdal is chopped liver, but that this wonderful new, breakthrough drug, Invega-- now, that's what's really best.
The Medical Letter offers its opinion: "[Invega] is likely to be similar to risperidone [Risperdal] in effectiveness and adverse effects. No specific madvantages of the new formulation have been demonstrated. ... Whether risperidone or [Invega] is a better choice than a first-generation drug that vosts much less is unclear." That last sentence is key. Research is now accumulating that suggests that the expensive second-generation antipsychotics like Risperdal probably have few of the advantages over the cheaper, older drugs that were originally attributed to them--another victory for pharmaceutical marketing over science.