Lomustine (CCNU) for Dogs: What It Treats and Why the Liver Gets Watched So Closely

Lomustine, almost always called CCNU, is an oral chemotherapy drug used in dogs for mast cell tumors, relapsed lymphoma, histiocytic sarcoma, and tumors in the central nervous system. It works, which is why oncologists reach for it often. It also carries two toxicities that shape every part of how it is monitored: it suppresses the bone marrow, and in dogs specifically, it can injure the liver.

Most owners hear about the liver first, and most of what they hear is only half the story. Elevated liver enzymes on bloodwork are common with this drug and usually do not mean liver failure. But the serious form of lomustine liver injury tends to show up late, sometimes weeks or months after the last dose, it builds with the total amount of drug a dog has received, and in dogs who develop it, it is often permanent. That is a different thing from a number ticking up on a chemistry panel, and the difference is worth understanding before you are asked to make a decision about it.

The toxicity more likely to land your dog in an emergency room in the first two weeks is a low white blood cell count. That one gets far less attention in owner-facing writing than it deserves.

Key Points

Lomustine is given by mouth and reaches the brain, which is why it appears in protocols for central nervous system tumors as well as for lymphoma, mast cell tumors, and histiocytic sarcoma (Barber & Burgess, 2023).

Low neutrophil counts are the acute dose-limiting problem. Neutropenia was documented in 56.9% of dogs in one large retrospective series (Heading et al., 2011), and a fever during the low-count window is an emergency, not a wait-and-see.

Liver enzyme increases are common. Roughly half of dogs show elevated ALT during treatment (Heading et al., 2011), and major elevations were reported in 29% of dogs in a separate series (Hosoya et al., 2009).

Clinical liver disease is much less common but far more serious. Hepatic toxicity affected 6.1% of dogs in one study, appeared a median of 11 weeks after the last dose, and was described by the authors as delayed, cumulative, irreversible, and potentially fatal (Kristal et al., 2004).

Severe enzyme elevations do not always creep up gradually. In 53% of the major elevations Hosoya et al. (2009) documented, there was no preceding mild rise to warn anyone.

There is one randomized trial supporting a hepatoprotective supplement during CCNU, and it studied a specific combination product rather than milk thistle alone. It measured enzyme values and treatment completion, not prevention of liver failure (Skorupski et al., 2011).

Ask your oncologist before adding anything, including supplements marketed as liver support.

What Lomustine Is and What It Does

Lomustine belongs to a class called alkylating agents, and within that class to a subgroup called the nitrosoureas. Alkylating agents bind directly to DNA and create the kind of damage that stops a cell from copying itself. They do not wait for a particular phase of the cell cycle, which is part of why they work against cancers that are not uniformly dividing at any given moment.

What sets lomustine apart from most chemotherapy drugs is that it is highly fat soluble and crosses the blood-brain barrier. Most drugs cannot get there. Its listed indications include lymphoma, mast cell tumors, histiocytic sarcoma, tumors of the central nervous system, and multiple myeloma (Barber & Burgess, 2023). In practice, dogs most often receive it for relapsed or resistant lymphoma, for mast cell tumors that cannot be handled with surgery alone, and for histiocytic sarcoma.

It is given by mouth. That makes it easier on everyone in some ways and creates its own responsibilities, since a cytotoxic drug is now in your house. Handling instructions matter, and they are worth going over with your team rather than improvising. Our guide to chemotherapy safety for the rest of your household covers the practical side of that.

The Toxicity That Comes First

Bone marrow suppression is the most common dose-limiting toxicity across conventional chemotherapy drugs, and lomustine is no exception. Neutrophils, the white cells that fight bacterial infection, drop after a dose and then recover.

In the Heading et al. (2011) review of 206 dogs treated with CCNU, 56.9% experienced neutropenia at some point during treatment, 34.2% became anemic, and 14.2% developed thrombocytopenia, meaning low platelets. Gastrointestinal upset showed up in 37.8%, most often as vomiting.

Timing varies. Neutropenia commonly occurs five to seven days after treatment with many chemotherapy drugs, but with lomustine and carboplatin it can occur as late as three weeks out, and in cats the delay can stretch further (Barber & Burgess, 2023). This is exactly why your team schedules a recheck bloodwork appointment at a specific interval after treatment rather than picking a convenient day.

Here is the part to carry with you. A dog with a low neutrophil count and a fever is a genuine emergency. You cannot measure a neutrophil count at home. You can measure a temperature, and a thermometer is the single most useful thing you can own during chemotherapy, which is why it heads our list of supplies to have before treatment begins. If your dog is febrile during the window your oncologist identified, call, and say the word chemotherapy when you do.

Thrombocytopenia with lomustine can also accumulate over repeated doses rather than resolving fully between them, which is one of several reasons bloodwork continues even when a dog looks perfectly well.

What the Liver Numbers Actually Say

This is where the popular version of the story goes wrong in both directions, understating the seriousness of clinical liver disease while overstating how alarming a raised enzyme is.

Enzyme elevations are common. In the Heading et al. (2011) series, 48.8% of dogs had elevated ALT at some point. Hosoya et al. (2009) looked specifically at how high those values went in 109 dogs on single-agent CCNU and found major elevation, defined as more than five times the upper reference limit, in 29%. Those elevations most often appeared after the first through third doses. In the untreated comparison group of the Skorupski et al. (2011) trial, 84% of dogs on CCNU alone showed some increase in liver enzyme activity.

Clinical liver disease is a different and much less frequent event. Heading et al. (2011) reported hepatic failure in 1.2% of their dogs. Kristal et al. (2004), reviewing 179 tumor-bearing dogs treated over six years, found hepatic toxicity in 11 dogs, or 6.1%.

That second study is the one worth reading carefully, because it describes the shape of the problem rather than only its frequency. The dogs who developed liver toxicity had received more doses and a higher cumulative amount of drug than the dogs who did not. The median time from the last dose to detection of liver toxicity was 11 weeks, with a range from 2 weeks all the way out to 49. Six of the affected dogs developed ascites, meaning fluid accumulating in the abdomen. Seven were euthanized because of progressive liver failure, with a median survival of nine weeks from the point of diagnosis. In the three dogs whose clinical signs did resolve, the abnormal bloodwork and the changes on biopsy persisted for anywhere from 4 to 38 months afterward. The authors concluded that CCNU can cause delayed, cumulative, dose-related, chronic hepatotoxicity that is irreversible and can be fatal (Kristal et al., 2004).

One more finding deserves emphasis because it undercuts a natural assumption. Owners tend to imagine liver values climbing slowly, giving everyone time to react. In 53% of the major elevations Hosoya et al. (2009) recorded, there was no preceding mild elevation at all. The value was normal, and then it was not. Younger dogs, five years and under, were at higher risk for the most severe elevations in that cohort.

None of this means most dogs on lomustine develop liver failure. Most do not. It means the monitoring is not a formality, and it explains why your oncologist may want bloodwork after treatment has finished.

Lomustine has also been associated with kidney effects and, with large cumulative amounts, lung toxicity (Barber & Burgess, 2023). Potential renal toxicity was reported in 12.2% of the dogs in the Heading et al. (2011) series. These are less common than the marrow and liver effects but are part of why the full picture gets checked rather than just a single value.

Why Bloodwork Happens Before Every Dose

Your team is asking two questions at each visit. Can this dog's bone marrow tolerate another dose right now, and is the liver showing signs of injury that would make continuing unwise.

If liver values rise meaningfully, the usual responses are to delay the next dose, reduce the amount given, or stop the drug. A pause is not a failure and it is not your team giving up. Given that the injury is cumulative and can be permanent, stopping is sometimes the decision that protects your dog's remaining time, even when the cancer was responding.

That trade-off is real and it is uncomfortable. It is also a conversation you can have in advance, before anyone is looking at a bad number, which tends to make it easier.

Hepatoprotective Supplements: What the Evidence Supports

You will see milk thistle and SAMe recommended widely for dogs on lomustine, often with more confidence than the research earns. Here is what actually exists.

Skorupski et al. (2011) ran a prospective randomized trial in tumor-bearing dogs receiving CCNU. Dogs were assigned either to receive a combination product containing S-adenosylmethionine and silybin, a bioavailable component of milk thistle, alongside chemotherapy, or to receive chemotherapy alone. Increased liver enzyme activity occurred in 84% of the dogs on CCNU alone and 68% of those receiving the supplement. Dogs on CCNU alone had significantly greater increases in ALT, AST, ALP, and bilirubin, and were significantly more likely to have treatment delayed or discontinued because of rising ALT.

That is a real result from a real randomized trial, and it is the reason many oncologists recommend hepatoprotective support during CCNU. It also has limits that get dropped when the finding travels.

The product tested combined SAMe and silybin. The trial cannot tell you what milk thistle does on its own, and it was not designed to. Describing milk thistle as the proven protective agent here goes beyond what was measured.

The outcomes were liver enzyme values and whether dogs finished their prescribed course. No dog in the study was shown to have been spared liver failure, and survival was not the endpoint. Whether lowering enzyme values reflects less underlying injury or only a lower number on the report is not something this trial settles.

The comparison group received nothing rather than a placebo, and the study was funded in part by the manufacturer of the product tested, with one author employed by that company. The investigators disclosed this openly, which is how it should work. It is still context you deserve when weighing the finding.

We go deeper into this in our article on milk thistle for dogs with cancer. Looking more broadly, a review of milk thistle use across farm and companion animals noted that information on its hepatoprotective properties specifically in dogs and cats remains limited, in part because studies in pets are difficult to run (Tedesco & Guerrini, 2023).

Where that leaves you is somewhere reasonable rather than dramatic. Asking your oncologist about hepatoprotective support during lomustine is a sensible question with an actual trial behind it. Starting a supplement on your own is not, both because some supplements interact with chemotherapy and because your team's read on your dog's bloodwork assumes they know everything your dog is receiving.

What to Watch at Home

The point of home observation is not to catch things earlier than the bloodwork does. It is to catch the things bloodwork cannot see between visits.

Take your dog's temperature on the schedule your team gives you, particularly during the days they flag as the risky window. Look at the gums and the whites of the eyes for a yellow tint, which can indicate jaundice. Notice appetite honestly rather than optimistically, since a dog who eats but leaves food behind is telling you something. Watch energy, and separate the ordinary quiet day after treatment from a dog who cannot be roused to normal interest. Note any vomiting or diarrhea, with dates and how many times. Check for pinpoint bruising on the gums or belly, which can accompany low platelets. And watch for a swollen or distended abdomen, which was a feature in six of the eleven dogs with hepatic toxicity in the Kristal et al. (2004) series.

Write it down. A dated note beats memory in an exam room every time, and your observations are information your oncologist has no other way to get.

When to Contact a Veterinarian

Call immediately, including after hours, for any of the following:

A fever, or any temperature reading above your veterinarian's stated threshold, especially during the days after treatment when counts are lowest. Fever in a chemotherapy patient is treated as an emergency until proven otherwise.

Yellowing of the gums, the whites of the eyes, or the skin.

Repeated vomiting, vomiting with blood, or diarrhea that is bloody or accompanied by weakness.

Collapse, extreme lethargy, pale gums, or a dog who will not stand.

A swollen or distended abdomen, or noticeable weight loss.

Pinpoint bruises, unexplained bleeding, or bleeding that will not stop.

Call during business hours for a decline in appetite even if mild, vomiting or diarrhea that is persistent but not severe, a dog who seems off in a way you cannot name, or any question about a medication or supplement. Appetite changes are worth reporting early. They are easier to manage at the start than after several days.

Ask your team directly what temperature they want you to act on and which days they consider the highest-risk window for your dog's protocol, and write both down somewhere you can find them at 2 a.m.

Questions to Ask Your Veterinarian

What is the goal of using lomustine for my dog, and what response would tell us it is working?

Which days after each dose are the highest risk for a low white cell count, and when do you want bloodwork done?

What temperature should prompt me to call you immediately, and what number should I call after hours?

What liver values are you tracking, and what change would cause you to delay, reduce, or stop treatment?

Will you continue monitoring bloodwork after the last dose, and for how long?

Do you recommend a hepatoprotective supplement during treatment, and if so, which product and why?

Is there a total cumulative amount of this drug you would not want to exceed for my dog?

How should I handle and store the medication at home, and what are the safety precautions for my household?

If we have to stop lomustine, what are the alternatives for my dog's cancer?

Sources

Barber, L. G., & Burgess, K. E. (2023). Overview of antineoplastic agents. Merck Veterinary Manual. https://www.merckvetmanual.com/pharmacology/antineoplastic-agents/overview-of-antineoplastic-agents (Full review April 2023; last updated September 2024)

Heading, K. L., Brockley, L. K., & Bennett, P. F. (2011). CCNU (lomustine) toxicity in dogs: A retrospective study (2002-07). Australian Veterinary Journal, 89(4), 109-116. https://doi.org/10.1111/j.1751-0813.2011.00690.x

Hosoya, K., Lord, L. K., Lara-Garcia, A., Kisseberth, W. C., London, C. A., & Couto, C. G. (2009). Prevalence of elevated alanine transaminase activity in dogs treated with CCNU (lomustine). Veterinary and Comparative Oncology, 7(4), 244-255. https://doi.org/10.1111/j.1476-5829.2009.00197.x

Kristal, O., Rassnick, K. M., Gliatto, J. M., Northrup, N. C., Chretin, J. D., Morrison-Collister, K., Cotter, S. M., & Moore, A. S. (2004). Hepatotoxicity associated with CCNU (lomustine) chemotherapy in dogs. Journal of Veterinary Internal Medicine, 18(1), 75-80. https://doi.org/10.1111/j.1939-1676.2004.tb00138.x

Skorupski, K. A., Hammond, G. M., Irish, A. M., Kent, M. S., Guerrero, T. A., Rodriguez, C. O., & Griffin, D. W. (2011). Prospective randomized clinical trial assessing the efficacy of Denamarin for prevention of CCNU-induced hepatopathy in tumor-bearing dogs. Journal of Veterinary Internal Medicine, 25(4), 838-845. https://doi.org/10.1111/j.1939-1676.2011.0743.x

Tedesco, D. E. A., & Guerrini, A. (2023). Use of milk thistle in farm and companion animals: A review. Planta Medica, 89(6), 584-607. https://doi.org/10.1055/a-1969-2440