**Thesis:** Peptides are a programmable middle layer between small molecules and biologics, and the history of the field is the history of paying down the *peptide tax* — proteolysis, rapid clearance, membrane impermeability, and weak oral exposure — with medicinal chemistry, formulation, and delivery engineering. # Intro — The Programmable Middle Layer - Frame peptides as a recognition layer sitting between small molecules and biologics, and state the three-axis thesis: target geometry × PK engineering × manufacturability. # From Hormone Analogs to a Modality - Why peptides spent decades boxed in as endocrine analogs, and how the peptide tax (proteolysis, clearance, impermeability, oral exposure) defined the reachable target space — with biologics as the first comparator anchor. # Paying Down the Peptide Tax - The medicinal chemistry moves that made modern peptide PK practical — backbone modification, D-amino acids, cyclization, PEGylation, fatty-acid derivatization, and permeation enhancers — each buying back stability, half-life, or oral exposure. # When Peptides Win — A Three-Axis Framework - A framework for modality choice along target geometry, PK engineering, and manufacturability, with RNA/oligo medicines as the foil for the geometry and delivery axes. # The GLP-1 Proof - Incretin-based therapies as the commercial proof that peptides can be mass-market drugs, cashing in directly on the fatty-acid derivatization and oral-delivery engineering from the previous sections. # Expanding the Target Surface — Cyclic Peptides and Conjugates - Whether constrained cyclic peptides, macrocycles, and peptide-drug conjugates represent a genuine expansion of addressable target space (PPIs, intracellular access, tumour penetration) versus a parallel improvement track, with ADCs as the PDC foil. # Manufacturing is the Other Bottleneck - Why synthesis is load-bearing for the thesis: SPPS aggregation, sequence-dependent failures, and the shift toward enzymatic and ligase routes as a supply-side constraint on which peptides can actually be made at scale. # The Orforglipron Tension - The Foundayo (orforglipron) approval on Apr 1, 2026 as the closing tension: peptides may be the modality that first turns hard biology into a druggable market, but once the biology is validated, small molecules can come after the same axis with a different convenience and cost profile. # References