Drug repurposing and the prior art patents of competitors

•Drug repurposing is dedicated to find novel indications for established substances.•Inside a drug class, prior patents may already cover ‘new’ indications, challenge patentability.•Novel formulation/dosing of established substances during repurposing could help overcome prior art.•Patentees file such novel patents even in absence of knowledge about competitors’ filings.•Many repurposing patents still fail due to other reasons than non-novelty of the new indication. Drug repurposing (i.e., finding novel indications for established substances) has received increasing attention in industry recently. One challenge of repositioned drugs is obtaining effective patent protection, especially if the ‘novel’ indications have already been claimed by competitors within the same drug class. Here, I report the case of patents relating to phosphodiesterase type 5 (PDE5) inhibitors. Patentees of later-filed patents on novel indications (even when they could not observe prior patenting of their direct competitors) filed patents for which patent examiners did not see the prior-filed patents of the competitors as relevant prior art, whereas these follower patent applications often failed because of other reasons.