History is replete with examples of compounds that were originally developed for one disease and subsequently found to be beneficial in another. In contrast to the hypothesis-driven philosophy of modern drug discovery, many highly successful new treatments have been discovered by serendipity [3,5,12-14]. The phosphodiesterase^ (PDE-5) inhibitor, sildenafil for example was originally developed as a potential anti-angina therapy but was observed during early clinical trials to be efficacious for male erectile dysfunction, for which it was subsequently first approved. Further studies on sildenafil have expanded its label to include approval for pulmonary arterial hypertension. The alpha-2 adrenergic agonist, brimonidine, was originally synthesized as an anti-hypertensive and later discovered and marketed as an anti-glaucoma agent. Further examples of drugs with unexpected benefits beyond their initially approved indications include: bupropion, which is approved as a smoking cessation drug, was originally developed and approved the treatment of depression ; gabapentin, which was originally launched for treatment of epilepsy but has also been extensively studied for several other indications including neuropathic pain ; eflornithine, an anti-trypanosomal drug, which was approved for the new indication of reducing unwanted facial hair [17,18]; finasteride, a type II 5-alpha-reductase inhibitor, which was originally approved as a treatment for benign prostate hyperplasia and then developed as a treatment for alopecia [19,20].
Far from being rare occurrences, additional indications for existing medicines are common. Indeed, additional indications can help contribute to the 'blockbuster' sales of a drug . Two surveys of blockbuster drugs have revealed that up to 40% of revenue can be derived from secondary indications to that which the drug was originally invented. Gelijns et al.  examined the top 20 selling US blockbuster drugs of 1993 and found that 40% of the revenues came from sales for secondary/ alternate indications. Ninety percent of the top 20 blockbusters were reported to have sales for secondary indications. In a similar analysis of the top 50 selling drugs in the UK in 1999 Pritchard et al.  found overall that only 62% of revenues were for the original indication and in total 25% of sales were for new or unlicensed ('offlabel') indications rather than the originally launched indication. The remaining 13% of prescriptions were classified as unknown but many of these may probably be for secondary indications. About half of the drugs examined in this survey had sales for additional indications.
The contributions to the medicine that are made by new indications is demonstrated by the number of new therapies approved for marketing. In recent years the number of therapies being marketed for new indications per annum is rising and is greater than the number of new therapies arising from first-in-class drugs reaching the market each year (Fig. 1). Around 10% of all new drugs launched each year target new mechanisms, about the same as the number of drugs approved with new indications.
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