Why a promising human medicine doesn't automatically become a treatment for pets

For many families, the hardest conversation in veterinary medicine comes right after a diagnosis. A dog has an aggressive cancer, or a cat's kidneys are failing, and the veterinarian has to explain that there is no approved treatment that can change how the disease progresses. The vet may be able to offer a human medicine used off-label, without dosing or safety data for that species, or may have no treatment to offer at all. Too often, the conversation ends with euthanasia. For many of these diseases, human medicine already has treatments that act on the same biology.

The distance between a drug that exists and a drug that is ready for pets helps explain why dogs and cats have far fewer approved medicines than people do. FDA's human drug center approved 46 novel drugs in 2025, according to the agency, close to its 10-year average. Its Center for Veterinary Medicine (CVM), which regulates animal drugs, typically approves fewer than 10 new animal drugs a year across every species it regulates, according to a 2026 review in the journal Veterinary Sciences. This includes livestock, fish and other animals in addition to companion animals like cats and dogs.

The humanization of pets, the shift toward treating dogs and cats as family, has made this gap more visible. Families increasingly expect the kind of care for their pets that they would want for themselves. In a 2023 Pew Research Center survey, 97% of U.S. pet owners said their pets are part of the family, and the American Pet Products Association estimates that Americans spent $158 billion on their pets in 2025. Companion animals account for about 70% of revenue at Zoetis, the largest animal health company. Interest in how long pets live is growing too, and a drug intended to extend the lifespan of senior dogs is partway through FDA review. For a family facing a pet's diagnosis today, though, the more pressing need is usually a medicine that changes the course of a specific disease, such as cancer, kidney disease or heart disease.

A drug developed for people is a strong starting point. Turning it into a veterinary medicine still requires its own studies in dogs or cats, manufacturing work and a dedicated regulatory path through the FDA CVM. Often the hardest problem is ownership: someone has to fund and run the veterinary development, and that is where many promising drugs stall.

Can veterinarians prescribe human medicines for dogs and cats?

Yes, within limits. The Animal Medicinal Drug Use Clarification Act of 1994, known as AMDUCA, lets veterinarians prescribe approved human and animal drugs in ways their labels don't cover, within a valid veterinarian-client-patient relationship. This is called extralabel use. Before AMDUCA, using a drug in any way that differed from its label was illegal.

Extralabel prescribing is a normal part of veterinary practice, and for many serious conditions it is the only option. Often, though, it puts the burden on the individual veterinarian. A human label describes doses, interactions and side effects in people. For a dog or cat, the vet has to fill in the rest from case reports, small studies and experience. General practitioners do this every day, usually without the time or case volume that a specialist can draw on.

What has to happen between a human drug and a pet medicine

In most cases, five things have to come together.

1. The disease biology has to match.

Many serious diseases in pets closely resemble their human counterparts. Dogs develop lymphoma, osteosarcoma, high-grade glioma and other cancers on their own, and the National Cancer Institute estimates that about 6 million dogs are diagnosed with cancer in the United States each year. The overlap goes well beyond cancer. Dogs get atopic dermatitis driven by many of the same immune signals as the human condition, and spinal disc disease that closely resembles disc degeneration in people. Cats develop chronic kidney disease and hypertrophic cardiomyopathy, which in some breeds is linked to mutations in a gene that also causes the human disease.

A similar-looking disease is only a starting point, though. The drug's target, usually the protein it binds, has to exist in the dog or cat in a form the drug still recognizes, and it has to play the same role in the animal's disease. Small differences in a protein's sequence can weaken or prevent binding of the drug.

2. The drug has to behave well in a dog's or cat's body.

Pharmacokinetics, the way a body absorbs, distributes, breaks down and clears a drug, can differ sharply between species. Cats lack a working version of UGT1A6, a liver enzyme people use to clear many drugs, which is why acetaminophen, a routine painkiller in people, is dangerous to cats. Many collies and other herding breeds carry an MDR1 mutation that disables P-glycoprotein, a pump that stops drugs accumulating in the brain and helps the body excrete them. Those dogs can react severely to ivermectin, loperamide and some chemotherapy agents.

Dose also doesn't scale neatly with body weight. Converting a dose by kilograms can overdose or underdose patients at either end of a range that runs from a 2-kilogram Chihuahua to a 70-kilogram mastiff. The right dose has to be worked out in the species itself, through dedicated pharmacokinetic and safety studies.

3. Antibody drugs often have to be rebuilt.

Many modern cancer and immune treatments are monoclonal antibodies. A dog's or cat's immune system can recognize an antibody from another species as foreign and make anti-drug antibodies against it, which can blunt the treatment or cause reactions. Drug developers reduce that risk by "caninizing" or "felinizing" the antibody, keeping the small regions that bind the target and rebuilding the rest from dog or cat antibody sequence. The first monoclonal antibody FDA approved for any animal, for osteoarthritis pain in cats in 2022, was made this way.

4. It has to become a product vets and pet owners can actually use.

That can mean a chewable tablet a dog will accept, a format that works for a cat who refuses pills, and strengths that cover the full range of body sizes. The finished product has to be manufactured to FDA standards, with stability data showing it keeps its quality over its shelf life. This is where an existing human program can help most, because the active ingredient may already be made at scale with a well-understood process.

5. The FDA CVM needs its own evidence.

Animal drugs are reviewed by CVM, and a human approval doesn't ensure that the drug will be safe or effective in dogs or cats. A sponsor opens an investigational new animal drug file, or INAD, and builds the case through a series of technical sections. The central ones are target animal safety, which shows the drug is safe in that species at the labeled dose; effectiveness, usually shown in well-controlled studies in pets with the disease; and chemistry, manufacturing and controls.

Conditional approval offers a second route for some programs. It was created for minor species and uncommon diseases and, since 2018, also covers serious conditions in dogs and cats when effectiveness studies would be especially difficult. A conditionally approved drug must meet full safety and manufacturing standards, and show reasonable expectation of effectiveness. The sponsor can market it for up to five years while it completes the remaining effectiveness work.

Why so many promising drugs never reach pets

None of these steps are unusual, and veterinary drug developers carry them out routinely. What usually holds a drug back is where its owner's attention is. For a drug developed for people, that owner is usually a human pharmaceutical or biotech company whose scientists, budgets and investors are focused on human patients. Developing the drug for dogs and cats isn't anyone's job there, and most human pharmaceutical companies have no veterinary team. A drug can have a validated target, years of human safety data and even a published study in pet dogs, and still never reach a veterinary clinic.

What it looks like when it works

Rabacfosadine is one example. Gilead Sciences developed it, as GS-9219, as a potential treatment for human blood cancers. Because the compound breaks down too quickly in rodent blood to be tested well in mice, Gilead studied it in pet dogs with lymphoma, where it showed activity. Results in human patients were less encouraging and the human program was discontinued, but Gilead licensed the molecule to a veterinary company, VetDC. As Tanovea, it received conditional FDA approval in 2016 and full approval in 2021, the first conditionally approved drug for dogs to reach full approval.

Verdinexor shows how veterinary research can feed back into human medicine. Karyopharm Therapeutics developed it alongside selinexor, a closely related drug for human cancers. What veterinary oncologists at Ohio State learned about dosing and side effects in dogs with lymphoma helped shape the human selinexor trials, and FDA approved selinexor for multiple myeloma in 2019. Verdinexor, sponsored for dogs by Anivive Lifesciences, received conditional approval for canine lymphoma in 2021 and has since received full approval.

Our co-founders, Elizabeth Iorns and Dan Knox, saw the same pattern before they started Familiar. Researchers at the University of Minnesota had developed a peptide that targets the CD200 immune checkpoint as a potential treatment for brain cancer in people, and tested a version of it in pet dogs with high-grade glioma, where dogs given it with a tumor vaccine lived longer than earlier dogs treated with the vaccine alone. No company had taken it forward for dogs. Elizabeth and Dan licensed it and partnered with Ardent Animal Health to pair it with Ardent's personalized cancer vaccine. Together they took the combination into a registrational study in dogs with osteosarcoma, an aggressive bone cancer, where it recently met its registrational endpoint. The project showed them that strong candidates exist and that someone has to carry them all the way to approval.

Why we started Familiar

Familiar was founded to do that work across many drugs at once. We partner with pharmaceutical and biotech companies to license the veterinary rights to drugs backed by extensive human research. Our partners keep the rights to each drug's human use and share in veterinary sales. We focus on serious diseases where nothing is approved, or where today's options manage symptoms without changing the course of the disease. We then take on the work described above, from confirming the biology and dose in dogs and cats to manufacturing, clinical studies with veterinary specialists and FDA review. Because we start from medicines with years of human research behind them, we aim to bring selected programs to pets significantly faster than the 7 to 10 years a new animal drug typically takes.

We now have 13 active programs, including four lead programs, across oncology, immunology and dermatology, neurology, and kidney and metabolic disease. We have signed two partnership and licensing deals and opened two INAD files with FDA. Our first clinical study, a pilot at the University of Illinois led by Dr. Timothy Fan for dogs with hemangiosarcoma of the spleen, opens soon. It is part of FAM-010, our program working toward the first FDA-approved treatment for hemangiosarcoma in dogs.

What we hope comes next

Our hope is that, in time, developing a medicine for people will routinely include asking whether it could also help dogs and cats. When a breakthrough works in people, there should be a clear path for the same science to reach dogs and cats within a few years, with a label that tells a general practice veterinarian how to use it. Many owners already pay for advanced care. For many serious diseases, there is still no approved medicine for their vet to prescribe.

Human medicine stands to gain too. A 2013 analysis in Nature Reviews Drug Discovery found that 56% of drugs that failed between Phase II and submission failed for lack of efficacy. Pets with naturally occurring disease can give earlier evidence about whether a drug acts on a disease, as the verdinexor example shows.

If you are a veterinarian interested in future studies, or a company with a program that could help pets, we'd like to hear from you at hello@familiartx.com. You can also sign up below for occasional updates on our research.