Native peptides die in the bloodstream in minutes, chewed apart by proteases and flushed through the kidneys. Turning them into weekly or monthly drugs is a chemical-engineering exercise: lipidation and albumin binding, PEGylation and its alternatives, D-amino acid substitution, and backbone cyclization. What each trick actually does, what it costs, and how the modern blockbusters stack them.
Lipidation
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The Peptide Half-Life Problem