Why do some peptides become approved medicines while others don't?

Drug development is expensive (over $1 billion for a typical phase-3 approval) and that investment has to be recovered through commercial sales. A pharmaceutical company will only take a compound through phase-3 development if four conditions are met: the biology works (mechanistic validity and animal-to-human translation), the compound is patent-protectable (specific chemistry that gives market exclusivity), the indication is regulatorily clear (defined endpoint, established approval pathway), and the market is large enough or supported by rare-disease incentives.

Compounds that meet all four criteria reach approval. Semaglutide (novel Aib-2/fatty-acid chemistry, obesity market of hundreds of millions, defined weight-loss endpoint), tirzepatide (novel dual-agonist chemistry, same markets), tesamorelin (a specific approved-drug indication supported by orphan-drug incentives), bremelanotide (specific HSDD indication with defined endpoint), afamelanotide (rare disease with orphan-drug incentives) all cleared the four filters.

Compounds that fail one or more filters remain in the research space. BPC-157 has genuine biological interest but is a fragment of a naturally-occurring protein — not patentable in the way a novel small-molecule would be. Its broad tissue-repair biology also lacks a single defined regulatory endpoint that a phase-3 trial could be built around. This combination means no pharmaceutical sponsor has taken it into phase-3 development, not because the biology doesn't work but because the drug-development economics don't fit.

The honest reading is that regulatory approval reflects investibility as much as biology. Absence of approval is not evidence that a compound doesn't work; it may just be evidence that the compound didn't fit the drug-development apparatus.