Can an IBD Biologic Last Six Months?

Here are all seven. Five are antibody targets: TNF, the α4 and α4β7 integrins, and the IL-23 axis at two points (IL-12/23 p40 and IL-23 p19). The other two are the newer pills, JAK and S1P. Almost everything in the pipeline is a bet on either adding a target to that list or doing something better with one already on it.

One of those bets is duration. Several companies are trying to turn IBD biologics given every few weeks into treatments given every three, or perhaps six, months. For a patient on lifelong therapy, that is a real gain: a few injections a year instead of one every couple of weeks, and often better adherence as fewer doses get missed. It’s an attractive way to stand apart, but the term “long-acting” can mean at least three different things, and only one of them proves anything to a patient.

Say a company calls its antibody long-acting. That could mean any of three things:

1. The drug stays in the blood for a long time. That’s PK, the half-life.
2. The drug keeps blocking its target. That’s PD.
3. The patient’s IBD stays controlled until the next dose. That’s clinical durability.

Only the third one proves that a long dosing interval actually works. Keep those three apart and most of the confusion in this race disappears.

What sets an antibody’s half-life?

An antibody is a Y-shaped protein. Its two tips grab the target; its stem, the Fc region, is what sets how long the antibody lasts. Cells are constantly pulling antibodies in from the blood and breaking them down, but inside the cell a receptor called FcRn grabs the antibody by that Fc stem and carries it back out to the bloodstream before it can be destroyed. The more reliably that Fc–FcRn grip holds, the more often the antibody is rescued instead of degraded, so it survives longer.

Half-life-extension engineering seems to be almost entirely about strengthening that grip. A few amino-acid changes in the Fc (or the choice of antibody subclass) make FcRn hold on a little tighter in the acidic pocket inside the cell, then release cleanly once the antibody is back in neutral blood, so more of each dose is recycled rather than cleared. That single lever is what every “long-acting” antibody here is pulling. You will see it branded a few ways: most companies say “half-life-extended” (HLE); Xencor calls its version the Xtend Fc, but underneath it is the same trick.

The half-life is the time it takes the antibody’s concentration in the blood to fall by half. If an antibody has a 60-day half-life, roughly 50% is left at 60 days, 25% at 120 days, and 12.5% at 180 days. So a half-life is not an expiry date, and it is not automatically the dosing interval. That remaining 12.5% might be plenty if the drug is potent and the target-blocking bar is low, or far too little if active disease is churning out large amounts of target.

How long do today’s IBD biologics last?

Today’s IBD biologics don’t stay in the body all that long. Their half-lives run from about a week to about a month: roughly 8–9 days for the quick ones like infliximab and mirikizumab, about 26 days for vedolizumab. The exact number depends on the molecule and the dose. In practice that means an injection somewhere between every two weeks and every eight.

A better way to compare these drugs is to look at how much of each is left when the next dose is due: the trough. One half-life gone and half is left; two, a quarter; three, an eighth. If we estimate that trough for each drug, we see the ones that looked so different on half-life reach it with surprisingly similar amounts left. Ignore infliximab, which starts with a big IV loading dose.

The striking case is infliximab: an ~8–9 day half-life, yet maintenance every 8 weeks, by which point its level has fallen to around 1% of the peak. It works anyway because a high IV loading dose creates a large starting level. And a long half-life, conversely, buys nothing if the drug needs a high concentration to hold its target.

The long-acting IBD antibodies now in trials: mostly TL1A

The long-acting push is still early, and for now mostly a TL1A story. There are 6 purpose-built half-life-extended programs in the clinic (4 against TL1A, and one each against α4β7 and IL-23), plus one already terminated, and not one has passed Phase 2. They are chasing a target the conventional drugs have all but validated: 3 ordinary-schedule TL1A antibodies have reached Phase 3 (tulisokibart, duvakitug, afimkibart), and tulisokibart has already won its Phase 3 in ulcerative colitis, so the target is being proven out on normal dosing before any long-acting version has read out at all.

But a trial’s phase tells you how far along it is, not whether it tests what actually matters: whether the disease stays controlled all the way to the next dose.

Why is the long-acting wave concentrated on TL1A? It is an inflammatory signaling protein encoded by the TNFSF15 gene, acting through the DR3 receptor like an inflammatory volume control, and human genetics, human bowel tissue and clinical trials all point to it as an important IBD pathway (the hope that it also curbs fibrosis is interesting but not yet clinically proven). That evidence has made it the most crowded emerging mechanism in ulcerative colitis and Crohn’s. TheraRadar’s index tracks 7 TL1A antibodies across 6 sponsors and 36 trials, every one still pre-approval (six are standalone anti-TL1A antibodies; the seventh is a TL1A×IL-12/23 bispecific).

The table below is that newer wave, ordered by how much real patient evidence exists, with registered trials at the top and press-release-only programs at the bottom. The middle column is the reality check: what TheraRadar’s trial index actually shows for each program.

A few of these deserve a closer look. SPY001 targets α4β7, the axis vedolizumab already validated, and a well-worn one: vedolizumab is the single most clinically-tested biologic in TheraRadar’s index, with 65 IBD trials, more than infliximab’s 54 or adalimumab’s 50. That makes a long-acting α4β7 antibody a comparatively de-risked bet on the target. The efficacy evidence is thinner, though: the early UC readout came from a single-dose, uncontrolled study, and Spyre has shown prolonged receptor coverage in the blood but not six-month disease control. And a single antibody isn’t really Spyre’s play in any case: its SKYLINE-UC platform tests SPY001, SPY002 and the IL-23 antibody SPY003 both alone and in pairs, and a co-formulated SPY001+SPY002 (SPY120) (two separate antibodies in one shot, not a bispecific) aims to deliver two mechanisms in a single quarterly or twice-yearly injection.

SPY002 brings the same long-half-life engineering to TL1A. After a 100 mg dose, free TL1A stayed below the assay’s measurement limit for 20 weeks. That is meaningful PD, but not proof that all TL1A was blocked in bowel tissue or that UC stayed controlled for 20 weeks. XmAb942 is the cleanest public bridge from a measured human half-life (74 days) to a real quarterly protocol: its every-12-week maintenance arm is registered and enrolling, not yet read out.

The half-life leap is real

The experimental antibodies genuinely do stay around longer. Current IBD biologics cluster at roughly 8–26 days; the leading long-acting candidates report 56–85 days in healthy volunteers, a 3–5× jump. Whether that becomes a 3–5× longer dosing interval is the open question.

The trough, not the half-life

It comes down to the trough: how much drug is left when the next dose falls due. If we set infliximab’s big IV loading dose aside, today’s maintenance biologics almost all reach it with somewhere between a tenth and a half of their peak still on board. Mirikizumab and guselkumab run the deepest, at roughly 12% and 10%.

The counter-intuitive part is that even at a twice-yearly interval, the experimental antibodies would still be sitting at about 19–20% of peak when the next dose is due, a shallower drop than mirikizumab and guselkumab already tolerate every four to eight weeks. On the blood-level arithmetic alone, quarterly and even six-month dosing land comfortably inside the range current drugs already occupy.

So the hard question was never “can the molecule still be in the blood at six months?” The molecules almost certainly can. The hard questions are whether the target stays blocked and the disease stays controlled at that trough, especially in active IBD, where clearance itself changes.

Why active IBD makes this harder

A healthy-volunteer half-life is only the starting point. Active IBD can raise or destabilize antibody clearance through low albumin, protein loss through the bowel, inflammation, body size, and anti-drug antibodies. And blood concentration doesn’t tell you exactly how much drug reaches the inflamed bowel wall. So the useful question is not “can we still detect the antibody at six months?” but: just before the next dose, is enough drug reaching the bowel, is the target still blocked, and is the disease still controlled?

Induction and maintenance also have to be kept apart. A long-acting drug may still need IV loading or several early doses to get active disease under control, followed by infrequent maintenance. That is exactly the shape of the trials now recruiting: Spyre’s Phase 2 SKYLINE-UC and Xencor’s XmAb942 study both use IV induction followed by subcutaneous maintenance.

How you’d actually test a long-acting dose

So how do you prove a six-month dose? There are two honest ways to design the trial, and the programs split between them.

One is to design the trial so it tests the long interval directly: put patients on the quarterly or twice-yearly schedule from the start, then measure what matters (endoscopy, biomarkers and steroid-free remission) at the end of that interval. Xencor’s XmAb942 takes this route, with a registered 12-week maintenance arm. It answers the question directly, but at the cost of resting the readout on an interval that isn’t proven yet.

The other is to establish the drug on a conventional schedule first, show it controls disease, then lean on the PK and target-coverage data to justify stretching the interval in a later study. Most of the cohort is on this path. It carries less risk of a failed readout, but the six-month claim stays a projection until a dedicated long-interval trial runs.

Neither is wrong; they are different bets on sequence.

One long-acting program has already stopped

One of these programs has already stopped. A first-in-human study of BB-TL1A-VIAL-HLE, a half-life-extended anti-TL1A from Vial Australia (NCT07029971), was terminated during Phase 1, after just 16 participants. The public record doesn’t give a reason, so this is not evidence that the long-acting design failed. A Phase 1 can stop for money, strategy or slow enrolment just as easily as for the molecule.

So, can a dose last six months?

The likely first breakthrough is quarterly maintenance, not twice-yearly treatment. The half-life leap from ~8–26 days to ~56–85 days is a genuine PK achievement, but as the trough math shows, the PK was never the wall. The clinical revolution begins only when objective IBD control lasts all the way to the end of the promised interval.

And duration is only half the story. Tracking the long-acting programs kept surfacing a second way companies are trying to differentiate, not on how long a dose lasts but on how hard it hits, by combining targets to push efficacy. Roche’s TL1A×IL-12/23 bispecific and Spyre’s co-formulated SPY001+SPY002 (SPY120) are early bets that two mechanisms beat one. Whether that actually deepens remission, and what it costs in safety, is a separate question from the six-month one, and one for another brief to take a deeper dive on.