Target occupancy — how you define it can make or break your drug
Why 100% target occupancy can be misleading for soluble targets — total target accumulation reflects binding, not the free-target reduction that actually drives therapeutic effect, and how the two can diverge based on complex vs. target half-life.
A drug can hit 100% “target occupancy” and still fail. Here's why.
For soluble targets at low concentrations (think cytokines), the usual PD assay is total target. If two ascending doses show no further accumulation, it's tempting to call the target saturated.
But when an antibody targets a cytokine, the antibody's half-life is usually longer than the target's — so binding "stabilizes" the target, and total target accumulates above baseline. That accumulation proves binding is happening. However, it doesn't prove free target has dropped enough for a therapeutic effect.
Target occupancy = bound target / total target. The saturated accumulation level (relative to baseline) is roughly set by the ratio of the drug-target complex half-life to the target's own half-life. If your complex's half-life is 100x the target's, occupancy approaches 100%, while telling you almost nothing about how much free target actually dropped. And free target reduction is the effect you're actually chasing.
So what does this mean for clinical strategy? It depends on whether a free-target assay is even feasible — post-dose, free target may be too low to measure reliably. Total target is still useful: with the right PK/PD model, it can reveal real information about the target. But it isn't sufficient on its own to determine the right dose or the right outcome.
Free target reduction is the outcome that matters.
Some references in the first post below
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#PKPD #RO #DrugDevelopment #Biologics #TMDD
A non-exhaustive list of references for a deeper dive:
Stein, A. and Ramakrishna, R. https://doi.org/10.1002/psp4.12169
Dua P, Hawkins E, van der Graaf PH. doi: 10.1002/psp4.41
Kapitanov et al., doi: 10.3389/fbinf.2021.731340 — "Right Dose" section.
Agoram BM. doi: 10.1111/j.1365-2125.2008.03297
Lowe, P.J., et al. https://doi.org/10.1111/j.1742-7843.2009.00513.x
Chimalakonda AP et al. doi: 10.1208/s12248-013-9477-3.
Davda JP, Hansen RJ. doi: 10.4161/mabs.2.5.12833.
Mager DE, Jusko WJ. doi: 10.1023/a:1014414520282.
Kd ≠ IC50
Why Kd and IC50 aren't interchangeable — a real case where two molecules looked equivalent in one binding assay and 3x different in another, and how target concentration relative to Kd explains why.
A mistake I've seen more than once: Kd ≠ IC50
A well-known concept to most, but one that gets confused in practice — often leading to the wrong interpretation of an in vitro assay result.
The story: in an SPR assay, molecule A was 3x more potent than molecule B. The engineering team decided to affinity-optimize molecule B for use as the clinical candidate. But in a concentration-based binding assay (ELISA), molecules A and B looked equivalent. The team was confused — why keep optimizing B if they're the same? It turned out the target concentration in the assay was so high that affinity no longer mattered — both molecules simply hit IC50 at half the target concentration. After adjusting the assay's target concentration for the next run, the team moved forward with further affinity maturation on molecule B.
Binding assays need to reflect the affinity of the molecule — which means the target concentration needs to be in range of the KD. If target concentration >> KD, you fall into a concentration-dependent regime that no longer reflects true affinity.
Of course, there are plenty of caveats: mono- vs. multivalent binding, whether the recombinant SPR reagent reflects the natural target (which can vary by company and batch), avidity effects, and more. But whenever I have both SPR data and data from another assay (often on-cell binding), I do at least a back-of-envelope check to see if the results align — and whether we're in a concentration-dependent regime for the binding assay.
This matters in the clinic as well — the inherent target concentration and how it relates to the therapeutic's affinity can both determine the "danger" of TMDD as well as whether affinity maturation is necessary.
What am I missing? Do you have a preferred binding assay for in vivo translation?
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#PKPD #affinitymaturation #KD #IC50 #DrugDiscovery #DrugDevelopment #TMDD