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Two antibodies with the same KD can behave completely differently. Can you engineer the rates instead of the ratio?

Yes. KD is a ratio of two independent physical rates, and identical ratios can be built from very different components: a fast-on, fast-off binder and a slow-on, slow-off binder can report the same affinity and behave nothing alike in a tissue, a tumour, or an assay. Abwiz Bio selects for the rates themselves, targeting on-rate and off-rate independently by designing wash duration and soluble competition directly into the phage selection. Long, competed washes enrich slow-off clones; short-contact selection favours fast-on clones. You receive clones matched to a specified kinetic profile, with on- and off-rate data, not merely to a KD number.

Where the ratio hides the mechanism

Affinity is reported as a single number because equilibrium is easy to measure. But almost nothing in vivo happens at equilibrium. An antibody perfusing a tumour has minutes of contact, not hours; an antibody that must penetrate deep into tissue is held at the periphery precisely because it binds too durably at the first target it meets; a diagnostic reagent in a lateral-flow strip has seconds. In each case the relevant question is a rate, not a ratio.

• Slow off-rate gives durable target occupancy: sustained receptor blockade, long residence time, longer functional effect per dose. It also produces the binding-site barrier, where the molecule saturates the first cells it encounters and never reaches the rest.
• Fast on-rate means the molecule captures target during brief exposure — important for short-contact formats, rapid diagnostics, and any setting where the antibody is cleared before equilibrium could be reached.
• Fast off-rate is a deliberate design goal more often than people expect: it enables tissue penetration by allowing repeated bind-and-release, and it is central to serial engagement in T-cell engagers, where an arm that never lets go engages one target instead of many.

None of these can be specified by asking for a lower KD. They have to be selected for directly.

How kinetic selection works

Phage display is well suited to this because the selection step is itself a kinetic experiment; the variables that are usually treated as protocol details are exactly the ones that determine which rate is being enriched.

• Off-rate selection by wash design. Extending wash duration converts panning into a dissociation experiment: clones that release during the wash are lost, and only slow-off variants remain. Adding soluble antigen as a competitor during the wash prevents rebinding, so the selection reports true dissociation rather than apparent retention.
• On-rate selection by contact time. Reducing antigen contact to a brief window means only clones that associate quickly are captured, enriching fast-on variants that a long incubation would have allowed slow binders to hide among.
• Bidirectional selection. Because both stronger and weaker binders can be captured by running the combined library across fast and slow on- and off-rate conditions simultaneously, we can move a clone in either direction rather than only toward tighter binding.
• Designed diversity underneath. STEM™ libraries supply the sequence space: CDR positions are varied using human antibody amino-acid usage, guided by a local AlphaFold instance plus germline and structural bioinformatics, with thermostability and polyreactivity filters at every stage.
• Measurement on the delivered molecule. Selected clones are converted to IgG and characterized by BLI, so the kinetic constants you receive describe the final format rather than the phage.

Related engineering axes

Kinetic tuning is one of three services that let you specify a binding profile instead of maximizing a single number:

• Affinity attenuation — deliberately dialling affinity down into a defined window, for agonists and T-cell engagers where binding that is too strong narrows the therapeutic index.
• Avidity engineering — separating monovalent affinity from the valency gain that drives receptor clustering.
• pH-dependent binding — the limiting case of conditional kinetics, where off-rate is engineered to depend on the local pH so the antibody releases antigen in the endosome or the acidic tumour microenvironment.

Where higher affinity genuinely is the goal, the same platform has driven leads from 5.9 nM to 4.4 pM, about 1,300-fold improvement, with the resulting clones passing developability filters as they go.

You receive

• On-rate and off-rate kinetic data (BLI) for the delivered clones
• Clones matched to the kinetic profile you specify, delivered as a panel where a range is useful
• Comparison against the parent antibody
• Full VH/VL sequences and recombinant IgG
• No downstream royalties, stage-gated approval before each next step

Frequently asked questions

Can you engineer on-rate and off-rate independently?

Yes. The selection conditions determine which rate is enriched: extending wash duration with soluble antigen competition selects for slow off-rate, while restricting antigen contact to a brief window selects for fast on-rate. Because these are separate levers, the two rates can be targeted independently rather than only through their ratio.

Why not simply select for the lowest KD?

Because KD is a ratio and two clones with the same KD can have completely different rate constants, which produce different behaviour in tissue penetration, target residence time, serial engagement and short-contact assays. If the application depends on a rate, selecting on the ratio leaves the outcome to chance.

Why would I want a faster off-rate?

A very slow off-rate can cause a binding-site barrier, where the antibody saturates the first target cells it encounters and never penetrates the rest of the tissue. Faster release allows repeated bind-and-release and deeper distribution. Fast off-rate is also central to serial engagement in T-cell engager formats, where an arm that does not release engages one target instead of many.

What data do you deliver?

Association and dissociation rate constants measured by BLI on purified IgG, alongside the parent antibody for comparison, plus full VH/VL sequences. Where a kinetic range is useful we deliver a panel spanning it rather than a single clone.

Do I need to start from scratch?

No. Kinetic tuning starts from your existing lead sequence. Send the VH/VL amino-acid sequence and the kinetic profile your application requires to info@abwizbio.com for a scoped quote.