Bald Monkeys Grew Hair Back for Years After Treatment Stopped. The Prolactin Blocking Drug Behind It Just Raised $100M (Human Trials Next)

Men avoiding minoxidil and finasteride may soon have a third option, and it works through a completely different pathway than anything currently available.

The drug is called ABS-201, and it just attracted $100 million in funding, including $40 million from pharmaceutical giant Eli Lilly.

But there’s a catch that matters more than the investment: it hasn’t regrown a single hair on a human head yet.

Certified trichologist Bobby Spence recently broke down the science behind this experimental treatment, explaining why researchers believe blocking prolactin receptors could offer hope to people who can’t or won’t use existing treatments.

Why Researchers Are Looking Beyond DHT

For decades, drugs like finasteride and dutasteride have treated hair loss by targeting DHT, the hormone that shrinks follicles in androgenetic alopecia.

ABS-201 doesn’t touch DHT at all. Instead, it targets prolactin, a hormone most people associate with pregnancy and breast milk production.

Hair follicles carry receptors for prolactin, which means the hormone can signal directly inside the follicle and influence its behavior.

According to Spence, prolactin can act like a signal telling the follicle to wrap up hair production early.

Think of your hair follicle as a small factory that produces one strand of hair at a time. Prolactin can act like a signal telling the factory to wrap up production early.

When this signal keeps firing, follicles spend less time actively growing hair. Strands become shorter and thinner, and over time, follicles miniaturize.

How ABS-201 Is Designed to Work

ABS-201 is a monoclonal antibody engineered to bind to prolactin receptors on hair follicles.

By blocking those receptors, the drug aims to stop prolactin from delivering its “shut down growth” message, potentially allowing follicles to stay in their growth phase longer.

The mechanism is compelling on paper, but Spence emphasizes an important distinction: a plausible biological target isn’t the same thing as proven hair regrowth.

Right now, ABS-201 has the first part without the second.

What the Animal Data Actually Shows

In mouse studies, ABS-201 reportedly produced close to full hair regrowth within 22 days, while mice receiving 5% minoxidil reached only around a third of that level.

That sounds impressive until you look closer at the numbers.

Minoxidil performed unusually poorly in that particular study, which means comparing the two results directly can be misleading.

Spence cautions against drawing broad conclusions from animal data alone, especially when minoxidil’s performance was far below what’s typically seen in human trials.

I wouldn’t look at those numbers and conclude that ABS-201 is already better than minoxidil. It’s encouraging animal data, but that’s about as far as you can take it right now.

The Human Trial Hasn’t Answered the Hair Growth Question

Phase one clinical trials for ABS-201 focused on safety, not efficacy.

That means researchers were checking whether the drug is safe to use in humans, not whether it actually grows hair.

Despite the excitement around mouse data and the $100 million funding round, zero human hair growth results have been published.

Safety matters here for another reason beyond just tolerability. Prolactin plays roles throughout the body, including in reproductive function, metabolism, and immune regulation.

ABS-201 isn’t applied topically to the scalp. It’s designed to block prolactin receptors systemically, meaning it affects the entire body.

Understanding what happens when you interfere with that pathway matters just as much as whether hair grows back.

Why the Earlier Drug Fell Short

Before ABS-201, another company called Hope Medicine developed HMI-115, which also targeted the prolactin receptor.

HMI-115 produced only modest results in human trials, but researchers believe the problem wasn’t the prolactin pathway itself.

It was how much of it they managed to block.

The prolactin receptor may not behave like a dimmer switch, where blocking half produces half the effect. Because of how the signal gets amplified downstream, researchers think you may need to block a very high percentage of receptors before you trigger a meaningful biological response.

In animal testing, HMI-115 reportedly reached around 90% receptor occupancy. In humans, that number dropped to roughly 70%.

If the threshold for effect sits above 70%, HMI-115 could have been hitting the right target without blocking enough receptors to produce real hair growth.

How ABS-201 Aims to Cross That Threshold

Aclaris, the company behind ABS-201, is testing dramatically higher doses than Hope Medicine used with HMI-115.

While HMI-115 topped out at 240 mg, ABS-201 doses go as high as 1,800 mg.

The goal is to push receptor occupancy past whatever threshold earlier drugs may never have reached.

According to Aclaris CEO Sean McLean, phase two planning uses doses between 300 and 1,200 mg, all aimed specifically at achieving the high receptor occupancy that HMI-115 reportedly fell short of.

In this view, the mechanism was never the problem, the dose was.

Why Simply Lowering Prolactin Doesn’t Work

If prolactin matters so much for hair growth, why doesn’t just lowering it solve the problem?

Cabergoline, a drug that lowers prolactin levels in the bloodstream, has been available for decades. It’s inexpensive and taken as a pill.

But it isn’t known for regrowing lost hair.

The reason may come down to where prolactin is actually coming from. Follicles can produce prolactin locally, right inside the follicle itself.

Lowering what’s circulating in your bloodstream doesn’t necessarily stop the signal happening at the follicle level.

ABS-201 takes a different approach by binding directly to the prolactin receptor, blocking the signal from getting through in the first place.

The Potential for Less Frequent Dosing

In the first human safety study, ABS-201 had a reported half-life of at least 65 days.

If that holds up through later trials, the eventual treatment could potentially involve only a few injections per year instead of a daily pill.

McLean compared it to how drugs like Ozempic work for weight management: periodic injections with the hope of lasting effects afterward, rather than constant redosing.

But frequency only matters if the treatment actually changes what the follicle is doing.

What Lab Studies on Human Scalp Tissue Revealed

Researchers have tested this pathway on actual human scalp tissue kept alive in the lab.

When prolactin signaling was blocked, follicles stayed in their growth phase longer.

But one observation stood out even more: some thin, miniaturized vellus hairs reportedly began moving toward thicker, pigmented terminal hairs.

That’s the opposite direction hair loss normally takes.

Lab studies also showed blocking prolactin signaling was associated with protective effects on follicle stem cells, along with increased activity from growth factors including IGF-1 and FGF-7.

If that same effect eventually shows up in living patients, it would suggest more than just maintaining existing hair. It would point to actual follicle regeneration.

But human scalp tissue in a lab is not the same thing as demonstrating meaningful regrowth on someone’s head.

The Unexpected Discovery That Started It All

According to McLean, the discovery of ABS-201’s hair growth potential started somewhere completely unrelated: endometriosis research.

When researchers treated mice with an early version of the compound, they noticed an unexpected side effect—faster hair growth.

That led them to test it in stumptail macaques, a monkey species that naturally goes bald in a pattern similar to humans.

The results, if they hold up in people, explain much of the excitement.

Bald monkeys reportedly grew hair back. Some gray hair darkened. And in some animals, growth reportedly continued for years after treatment stopped.

That last part is what sets ABS-201 apart from most hair loss treatments, which only work while you’re actively taking them.

Could It Reverse Gray Hair Too?

In lab settings, the team reportedly observed increased melanin production, raising the possibility that gray hair could darken as a side effect of treatment.

This is very early and completely unconfirmed in humans, but if it holds up, it would represent a meaningful secondary benefit on top of density improvements.

Spence admits the claim surprised him at first but notes it’s still just a hypothesis at this stage.

The Role of AI in Designing ABS-201

Aclaris used machine learning models to sift through massive amounts of data on drug structures, protein interactions, binding strengths, and how long different molecules stay active in the body.

From there, the models identified antibodies designed to bind tightly to prolactin receptors, last longer in the body, and have a better chance of clearing that high receptor occupancy threshold.

AI didn’t discover the prolactin pathway, but it helped optimize the drug candidate targeting it.

Could Stress Be Feeding Into This Pathway?

McLean’s team also explored a connection between stress and prolactin signaling.

As stress raises cortisol, cortisol can raise prolactin. Higher prolactin may increase scalp inflammation and speed up follicle damage.

That means chronic stress could, at least in theory, feed into the exact pathway ABS-201 is designed to block.

It’s more hypothesis than established finding, but it’s an interesting thread connecting everyday lifestyle factors back to follicle biology.

What About Areas That Are Already Bald?

Most current treatments only work on follicles that are still alive but miniaturized, turning weak vellus hairs back into strong terminal hairs.

When asked whether ABS-201 could help areas that are already completely bald, McLean was cautiously optimistic rather than definitive.

If the drug’s effect really is regenerative, the hypothesis is that it could reactivate dormant follicles that current treatments simply can’t reach.

That would matter most for advanced Norwood 6 and 7 level baldness, assuming the scalp hasn’t undergone permanent scarring.

What to Expect for Efficacy and Cost

McLean’s own benchmark for expected results is fairly measured: somewhere around 30 to 35 additional hairs per square centimeter.

He compared that to what a strong minoxidil regimen can achieve, just potentially with a few injections per year instead of daily application.

But there’s another question that matters: cost.

ABS-201 is a monoclonal antibody, and treatments in this class can be very expensive to manufacture and administer.

If a future hair loss treatment ends up costing thousands of dollars per year, that’s a very different proposition from generic minoxidil or finasteride, especially when hair loss treatments are usually paid for out of pocket.

The Biggest Open Questions

Spence is clear about where the evidence stands right now.

As of right now, ABS-201 has not regrown a single hair on a single human being. The data that actually settles this, the hair growth results, is still months away.

The biological mechanism is real. Hair follicles do carry prolactin receptors, and there’s genuine published research suggesting that blocking that pathway could matter.

The safety data so far is clean, and the dosing strategy makes it clear Aclaris learned something specific from where the earlier drug came up short.

But none of that is proof.

Phase two trials will provide the first real answers about whether ABS-201 actually grows hair in humans, and those results are still months away at minimum.

What This Means If You’re Losing Hair Right Now

If what you’re doing right now is working for you, this isn’t a reason to change anything yet.

And if you’re thinking about putting hair loss treatment on hold until ABS-201 finishes its trials, Spence has a blunt reminder.

Your follicles aren’t going to pause and hold their current size for the next two or three years while this plays out. If they’re miniaturizing right now, that process keeps going.

For androgenetic alopecia, the two treatments with the most evidence behind them are still minoxidil and the DHT blockers finasteride and dutasteride.

Nothing about ABS-201 changes that, and nothing about it gives reason to set either one aside today.

But both drugs come with tradeoffs. Finasteride carries the risk of sexual side effects. Minoxidil can cause scalp irritation, unwanted hair growth elsewhere, and in some cases, lightheadedness or drops in blood pressure.

If neither has ever felt like the right fit, there are other evidence-backed options that don’t rely on hormones or pills, including low-level laser therapy, which has decades of research and FDA clearance behind it.

The Bottom Line

ABS-201 represents a genuinely novel approach to treating hair loss, and the science behind it is worth taking seriously.

But $100 million in funding and promising animal data don’t replace what’s still missing: proof that it works in humans.

If you’re curious where the future of hair loss treatment is heading, it’s absolutely worth keeping an eye on.

Just don’t let a funding round do the talking for a drug that hasn’t been tested for the one thing it’s supposed to do.