Everything That Can Go Wrong When Making Peptide Drugs — a Complete Impurity Guide

Peptide drug impurities fall into three categories: (1) synthesis-related impurities from solid-phase peptide synthesis (SPPS) — including amino acid deletions, insertions, racemization, incomplete side-chain deprotection, oxidation, and dimerization; (2) degradation products from chemical instability — including β-elimination, diketopiperazine formation, pyroglutamate formation, and succinimide formation; (3) finished product impurities from API-excipient interactions. Trifluoroacetate (TFA) counter-ion contamination from SPPS purification and cross-contamination with unrelated peptides (indicating inadequate GMP) were also documented.

Comprehensive literature review cataloging all known types of peptide-related impurities in both active pharmaceutical ingredients and finished drug products, organized by their origin: synthesis-related, degradation-related, or formulation-related.

With the peptide drug market growing twice as fast as other drug categories, understanding and controlling impurities is critical for patient safety. These impurities can affect drug efficacy, cause allergic reactions, or produce misleading results in early drug discovery. For compounded peptides (which lack the same quality controls as FDA-approved products), impurity risks are even greater. This review provides a comprehensive catalog of what can go wrong during peptide manufacturing.

This review is especially relevant today as compounding pharmacies produce peptides like semaglutide and BPC-157 without the same quality controls as FDA-approved manufacturers. Every impurity type described here can occur in any peptide production setting, but GMP-compliant pharmaceutical manufacturers have stringent testing to detect and control them. The compounding pharmacy debate often centers on exactly these quality concerns — whether the impurity profile of compounded peptides meets safety standards.

This is a review article from 2014 — newer synthesis techniques and quality control methods may have addressed some of the impurity challenges described. The review focuses primarily on SPPS-manufactured peptides, which is the dominant method but not the only one (recombinant peptide production has different impurity profiles). Specific impurity levels and acceptable thresholds vary by regulatory jurisdiction and aren't comprehensively covered.