Almost every sentence written about a research peptide contains a pharmacology term doing quiet, load-bearing work. Ipamorelin is described as "selective." Tirzepatide is described as "biased." AgRP is described as an "inverse agonist." Melanotan 2 is "nonselective," PT-141 is "MC4R-preferring." Those words are not decoration — each one is a specific, testable claim about how a molecule interacts with a protein, and each one is routinely used incorrectly in vendor copy and forum summaries.
This piece is the companion to our pharmacokinetics primer. That one covered what the body does to a compound — absorption, distribution, half-life, clearance. This one covers the other half: pharmacodynamics, what the compound does at the receptor. No new compounds are introduced here. The goal is that the next mechanism paper you open is readable.
Affinity, Efficacy, and Potency Are Three Different Things
When a source says a compound is "strong" or "powerful," it could mean any of three unrelated properties. Separating them is the single highest-yield move in reading receptor literature.
Affinity is how tightly the ligand binds. It answers only: does the molecule stick to the receptor, and how well? It is reported as a dissociation constant (Kd) or, from competition-binding experiments, an inhibition constant (Ki) — both in units of concentration (typically nanomolar or picomolar). Counterintuitively, lower numbers mean tighter binding, because the value represents the concentration needed to occupy half the receptors. A picomolar Kd is far tighter than a nanomolar one.
Efficacy is what happens after binding — how much biological response an occupied receptor produces. This is a completely separate axis. A molecule can bind superbly and do nothing at all.
Potency is the concentration required to produce a given effect, usually reported as EC50 (half-maximal effective concentration) or, for inhibitors, IC50. Potency is not a pure molecular property. It is a composite of affinity, efficacy, and features of the experimental system — how many receptors the cells express, how efficiently the receptor couples to its signaling machinery, which readout was measured.
The practical consequence: two EC50 values from two different papers are frequently not comparable, even for the same receptor. Different cell line, different assay, different number. Comparing potency across studies without checking the system is one of the most common errors in secondary literature.
The Agonist Family: Full, Partial, Antagonist, Inverse
Efficacy is a spectrum, not a switch, and the vocabulary maps onto that spectrum.
A full agonist binds and produces the maximal response the system can generate. Native ligands are usually — not always — full agonists at their own receptors.
A partial agonist binds and produces a submaximal response even when every receptor is occupied. Adding more compound will not get you to the ceiling; the limit is intrinsic to the molecule, not the concentration. Importantly, in the presence of a full agonist, a partial agonist can behave as a functional antagonist, because it occupies receptors that would otherwise produce a full signal. Partial agonism is a pharmacological property, not a grade — it is neither a weakness nor a selling point.
An antagonist binds without producing a response. It occupies the site and blocks it. Competitive antagonists compete with the agonist for the same (orthosteric) site and can be overcome by more agonist; non-competitive antagonists cannot.
An inverse agonist requires one more concept: constitutive activity. Many G-protein-coupled receptors signal at some baseline level with no ligand bound at all. The ghrelin receptor GHS-R1a is a well-documented example of a highly constitutively active receptor. An inverse agonist binds and pushes activity below that baseline — a negative-efficacy ligand. AgRP acting at MC3R and MC4R is the canonical endogenous example, discussed in our melanocortin system explainer. This is why "antagonist" and "inverse agonist" are not synonyms: an antagonist silences the ligand, an inverse agonist silences the receptor.
A fourth category sits off the main axis. Allosteric modulators bind somewhere other than the ligand's own site and change how the receptor responds to its native ligand — positive (PAM) or negative (NAM). Allosteric claims are also the most frequently over-asserted in secondary sources; when we covered Selank, the GABA-system evidence was explicitly held at possible modulation rather than demonstrated PAM activity, because the primary literature does not establish the binding site.
"Selective" Means Three Different Things on This Shelf
Selectivity is the most abused word in peptide marketing, largely because it describes at least three distinct phenomena that happen to share a label.
1. Receptor-subtype selectivity. The compound engages one receptor subtype and not its close relatives. This is the strict pharmacological meaning. Melanotan 2 is nonselective across MC1R, MC3R, MC4R and MC5R; PT-141 is described as MC4R-preferring. Same family, different breadth of engagement.
2. Downstream hormonal specificity. This is not receptor selectivity, and conflating the two produces a very common error. Ipamorelin is routinely called "the selective GHRP" relative to GHRP-6. But both act at the same receptor, GHS-R1a. The difference is in the hormonal profile downstream — the degree to which ACTH, cortisol and prolactin move alongside growth hormone. We made this correction in detail in the 2026 growth-hormone axis roundup after cryo-EM structures of GHS-R1a were published. "Selective" there is a phenotype, not a binding measurement.
3. Tissue distribution. A receptor expressed in one tissue effectively confines the ligand's action there, regardless of binding breadth. MC2R is the clean case: it responds only to ACTH, and it additionally requires the accessory protein MRAP to traffic and function at all.
That last point generalizes into something worth stating outright: "the receptor" is not always one protein. Amylin receptors are not a standalone gene product — they are the calcitonin receptor complexed with a RAMP (receptor activity-modifying protein), generating the AMY1/AMY2/AMY3 subtypes that cagrilintide research turns on, as covered in the metabolic class primer. When a paper says a compound is selective, the honest follow-up question is always: selective by which of these three measures, and measured how?
Same Receptor, Different Signal: Biased Agonism
For decades, receptor activation was modeled as a single dial. That model is obsolete. A GPCR can couple to multiple downstream pathways — classically G-protein signaling (such as cAMP production) and β-arrestin recruitment, which also drives receptor internalization and desensitization.
Biased agonism (functional selectivity) means a ligand preferentially engages one of those pathways over the other, relative to the native ligand. Two compounds can hit the same receptor with comparable affinity and produce meaningfully different cellular consequences because they favor different arms.
Tirzepatide is the reference example on this catalog: its activity at GLP-1R has been characterized as cAMP-favoring relative to β-arrestin recruitment, with reduced receptor internalization compared with semaglutide. We laid out that evidence in the semaglutide vs tirzepatide comparison. The literacy point here is narrower: a single potency number cannot describe a biased ligand, because the answer depends entirely on which pathway was measured.
Receptors Adapt, So Timing Is Pharmacology
Receptors are not passive switches; sustained stimulation changes them. GRK phosphorylation, β-arrestin recruitment, internalization and downregulation all reduce responsiveness over time. Desensitization (or tachyphylaxis) means the same exposure produces a diminishing response.
This makes the temporal pattern of exposure a pharmacological variable in its own right, not merely a scheduling detail. The clearest illustration on this site is the reproductive axis: pulsatile GnRH stimulates gonadotropin release, while continuous GnRH exposure desensitizes the receptor and suppresses it — the same molecule at the same receptor producing opposite directional outcomes purely as a function of delivery pattern. That is covered in the HPG axis explainer, which is also where gonadorelin sits.
The growth-hormone axis carries its own version through the somatostatin brake and endogenous pulsatility, discussed in the GH secretagogue axis explainer. It is also why pharmacodynamics and pharmacokinetics cannot be read separately — half-life determines exposure pattern, and exposure pattern determines whether a receptor keeps responding.
Why a Cell-Assay Number Doesn't Transfer to an Organism
A clean, low EC50 in a transfected cell line is a real measurement of a real property. It is also a measurement made in a system with no liver, no proteases, no plasma-protein binding, no blood-brain barrier and often a non-physiological density of receptors.
Three failure modes recur:
- Receptor reserve. Cell lines frequently overexpress the receptor. In a high-reserve system, a partial agonist can produce a full-looking maximal response. The same ligand in tissue with sparse receptors may not.
- Pharmacokinetics. Exquisite in vitro potency is irrelevant if the compound is cleared in minutes or never reaches the compartment where the receptor lives — see the PK primer.
- Species and ortholog differences. Receptor sequences differ between rodent and human, and affinity does not always carry across.
None of this makes in vitro pharmacology unreliable. It makes it specific: an EC50 is a statement about a molecule in a defined system, and treating it as a portable property of the molecule is where the reasoning goes wrong.
Short FAQ
Is a higher-affinity compound always more effective? No. Affinity and efficacy are independent axes. Pure antagonists can have very high affinity and produce no response at all.
Does "selective" on a product description mean receptor-subtype selective? Not reliably. It may mean subtype selectivity, downstream hormonal specificity, or tissue restriction — three different claims with three different kinds of evidence behind them. Check which one the primary literature supports.
What is the difference between potency and efficacy in one line? Potency is how much you need; efficacy is how high it can go. A compound can be extremely potent and still be a partial agonist with a low ceiling.
Receptor pharmacology vocabulary exists to make claims falsifiable. When a description survives translation into these terms — which receptor, what affinity, what efficacy, measured in what system — it is describing something real. When it dissolves, that is informative too. For compound-by-compound references see the peptide library, and for analytical standards see quality.
This article is educational and for the laboratory research community. Trulogic Labs products are sold for laboratory and research use only and are not for human consumption.