Chapter 29 Key Takeaways — The Peptide Drugs Already in Your Pharmacy

The one-sentence version

Peptide medicine is a mature field with more than eighty approved drugs in routine daily use, most of which nobody thinks of as peptides — and every one of them either replaces a missing signal, blocks an overactive one, acts locally, kills a pathogen, or asks a diagnostic question, which means the category of "peptide that broadly supports or optimizes a healthy system" is empty.

The inventory

• More than eighty peptide drugs are approved in major jurisdictions. Exact counts vary because the peptide/protein boundary is a convention rather than a chemical fact (Chapter 1 §1.5). Distrust any source that gives a precise number without stating its inclusion rule.
• The public conversation about peptides covers roughly five molecules at the famous end and a dozen at the gray-market end. The quiet middle is where most approved peptide medicine actually lives.
• Clinicians organize drugs by function, not by molecular class, which is correct for running a hospital and disastrous for reasoning about a category. Nobody says "hang the peptide."

Teriparatide — the chapter's best story

• Teriparatide is PTH(1–34), the N-terminal fragment of parathyroid hormone, retaining full receptor-activating capability at roughly 40% of the mass.
• Chronically elevated PTH removes bone — that is what hyperparathyroidism does. Intermittent, once-daily exposure builds it. Same molecule, same receptor, opposite outcome.
• The only variable that changed is the temporal pattern of exposure. This is Chapter 3 §3.5's pulsatility principle producing an approved therapy rather than merely a caution.
• Teriparatide is an anabolic agent — it adds bone. Antiresorptives (bisphosphonates, denosumab) preserve existing bone and dominate prescribing. Teriparatide is positioned for severe osteoporosis and high assessed fracture risk.
• Abaloparatide is a related PTHrP(1–34) analog acting at the same receptor.
• Labeling history: a boxed warning about osteosarcoma, derived from lifetime high-exposure rat studies, was present at approval and later removed after roughly two decades of human surveillance failed to show the expected excess. Structurally the same problem as Chapter 8's thyroid C-cell question: a real animal finding, a species with different relevant biology, a precautionary label, and human data eventually answering what the animal study could not.

Desmopressin — selectivity as a lever

• Vasopressin acts at V1 (vascular smooth muscle → vasoconstriction) and V2 (renal collecting duct → water reabsorption).
• Two modifications — deamination at position 1 and a D-arginine at position 8 — produced a molecule that strongly favors V2 over V1 and lasts substantially longer. The D-residue also resists peptidase clearance.
• Three approved indications from one selectivity change: central diabetes insipidus, primary nocturnal enuresis in children, and certain bleeding disorders — the last because V2 stimulation releases stored von Willebrand factor and factor VIII from vascular endothelium.
• Receptor subtype selectivity is not a refinement, it is a lever. It converts a hormone with a broad physiological footprint into a tool acting on one branch of it.
• Hyponatremia is the risk that matters, and it follows directly from what the drug is designed to do.

Calcitonin — an honest contraction

• Calcitonin is a 32-residue hormone from the thyroid C cells; the therapeutic agent is usually salmon calcitonin, which is more potent at the human receptor than the human sequence is.
• Its broad osteoporosis role contracted for three independent reasons: better-evidenced competitors arrived, the fracture evidence never got strong (the PROOF trial's dose arms did not line up, and attrition was substantial), and regulatory reviews in the early 2010s concluded the benefit-risk balance did not support the indication.
• No scandal, no suppression, no villain. This is the book's clearest example of a class shrinking because the evidence improved — the process the unregulated peptide market conspicuously lacks.
• A drug does not need to be disproven to lose its place. It only needs to be outperformed.
• A trial with multiple dose arms should be read for whether the arms line up, not just for which arm carries the p-value.

The critical care and rescue peptides

• Vasopressin is a peptide vasopressor in vasodilatory shock, acting through V1 — a non-adrenergic pathway that adds tone alongside norepinephrine. VASST and VANISH support an adjunct role; a general mortality benefit has not been convincingly demonstrated.
• Vasopressin was used in hospitals for decades as an unapproved marketed drug and only received formal FDA approval in the mid-2010s. Routine practice and recent approval are not incompatible.
• Terlipressin — longer-acting, relatively V1-preferring — is used in hepatorenal syndrome, approved in the US in the early 2020s.
• Glucagon is a 29-amino-acid peptide cut from the same proglucagon precursor as GLP-1 (Chapter 7), used as rescue for severe hypoglycemia (Chapter 11 §11.4).
• The glucagon delivery problem was the user, not the molecule. Old kits required reconstitution; the realistic administrator is a frightened bystander. Nasal and ready-to-use liquid formulations, available since 2019, eliminated the mixing step and substantially improved successful administration by untrained users. The molecule did not change. The number of ways to fail did.

The diagnostic peptides

• A stimulation test delivers the stimulus that should provoke a response and measures whether the response comes — more informative than an ambiguous baseline measurement.
• Cosyntropin is ACTH(1–24), used to assess adrenal responsiveness. GnRH and TRH were used historically and were displaced by better assays — diagnostic peptides get retired by improvements in measurement, not by being disproven.
• Secretin, the first hormone ever identified (Bayliss and Starling, 1902; Chapter 3), is now an approved diagnostic agent for pancreatic exocrine function.

The peptides nobody calls peptides

• Cyclosporine is a cyclic undecapeptide — eleven residues, closed ring, containing a D-amino acid and heavily N-methylated. Those features defeat exopeptidases, defeat protease recognition, and remove backbone hydrogen-bond donors, which is what makes it orally bioavailable and active at an intracellular target. It made modern organ transplantation practical.
• Vancomycin is a glycopeptide antibiotic on the WHO essential medicines list. Bacitracin, polymyxins, and daptomycin are peptides too (Chapter 25), and the recurring structural theme is cyclization — microorganisms solved peptide stability long before chemists reinvented it.
• Octreotide (Chapter 27) and oxytocin (Chapter 21) belong on the same list.
• Peptide medicine is not an emerging field. It is a mature field having a good decade — and "we're at the frontier" is the most effective available excuse for not having evidence.

The modern approvals

• Icatibant is a synthetic decapeptide antagonist at the bradykinin B2 receptor, for acute hereditary angioedema attacks. Peptide antagonists are less common and harder to design than agonists.
• Linaclotide is a 14-residue peptide taken orally and designed not to be absorbed, acting on the guanylate cyclase-C receptor on the gut's luminal surface. Plecanatide is related.
• A peptide's confinement to a compartment is a limitation only relative to a target outside that compartment. Move the target and the same property becomes an advantage. Oral vancomycin for C. difficile runs the same logic.

The payoff — and the prior

Sort every approved peptide drug by what it does:

1. Replace a missing or insufficient signal — requires a deficit
2. Block or suppress an overactive signal — requires an excess
3. Act locally in a defined compartment — the compartment does the targeting
4. Kill a pathogen — an external invader, not a human system
5. Ask a diagnostic question — no therapeutic intent
6. Broadly support, optimize, enhance, or modulate a healthy system — empty

• The regulatory-artifact objection is partly right and must be conceded: approval frameworks are built around disease. But the incentive to enter box six is enormous and unclaimed; adjacent approvable categories (prevention, risk reduction) exist and are also thin in peptides; and the same pattern independently holds in Chapter 27's oncology survey and Chapter 37's master table.
• The prior: a claim that a peptide will support, optimize, enhance, or modulate something in a person without an identified deficiency, excess, pathogen, or localized pathology is a claim of a type that has never once obtained approval — despite fifty years of development, enormous commercial incentive, and more than eighty approved drugs in the class.
• That is not proof any specific claim is false. It is a very strong prior, built from public facts, applicable without knowing anything about the specific molecule.

Ratings issued in this chapter

Note that desmopressin appears twice, with two separately reasoned ratings, and that calcitonin's ❌ attaches to one claim while leaving its other uses unrated. One molecule, many ratings — rule 6, doing exactly the work it exists to do.