Sulfur mustard, the notorious chemical weapon known as mustard gas, is not itself used in chemotherapy, but its close chemical relatives, called nitrogen mustards, have been cornerstones of cancer treatment since the 1940s. The connection is more than historical trivia. Observations of what mustard gas did to soldiers’ blood cells during two world wars directly inspired the first successful cancer drug, and several descendants of that original compound remain in clinical use today.
How a Wartime Poison Became a Cancer Drug
The path from chemical weapon to medicine began during World War I. Doctors treating soldiers exposed to sulfur mustard noticed something striking: the poison devastated fast-dividing cells, particularly in the bone marrow. Autopsies of soldiers who died from mustard gas exposure showed bone marrow that had undergone near-complete destruction of its blood-forming elements, leaving behind little more than a gelatinous matrix with dilated blood vessels and scattered bleeding. The lymph nodes and spleen suffered similar damage, which explained the severe drop in white blood cells observed in living patients.1PubMed Central. The Blood and Bone Marrow in Yelloe Cross Gas (Mustard Gas) Poisoning
This was a catastrophe for healthy soldiers, but it planted an idea: if a chemical could selectively destroy fast-dividing cells, perhaps it could be turned against cancers, which are essentially cells dividing out of control. That idea gained urgency during World War II. In December 1943, a German air raid on the Italian port of Bari caused over 100 tons of mustard gas to spill from a secret Allied supply ship into the harbor. Hundreds of servicemen and civilians were exposed. Medical investigators studying the casualties confirmed the same pattern of bone marrow and lymphoid tissue destruction seen in World War I. The accidental disaster at Bari became a catalyst for medical research that laid the foundation for chemotherapy treatments still in use decades later.2Military Medicine. The Bari Bombing and the Birth of Chemotherapy
Meanwhile, pharmacologists at Yale had already been quietly experimenting with nitrogen-based variants of mustard gas, compounds that retained the cell-killing power but were somewhat more manageable than the sulfur-based weapon. The first of these drugs, mechlorethamine, was tested on a patient with advanced lymphoma, and the tumor shrank dramatically. This was the first time a chemical agent had caused a cancer to regress. The results were classified as military secrets during the war and published only afterward, but the age of chemotherapy had begun.3PubMed Central. War! What is it good for? Mustard gas medicine
The Difference Between the Weapon and the Medicine
Sulfur mustard and nitrogen mustards are both alkylating agents, meaning they work by chemically attaching reactive groups to DNA, which gums up the cell’s ability to copy itself and eventually kills it. But they are not the same compound. Sulfur mustard contains a sulfur atom at its core. Nitrogen mustards replace that sulfur with a nitrogen atom, which changes the drug’s behavior in the body. Both are toxic to living cells, but nitrogen mustards were found to be more suitable for medical use because their chemistry could be tuned, making them more selective or directing them toward specific types of tissue.4PubMed. Comparative toxicity of sulfur mustard and nitrogen mustard on tracheal epithelial cells in primary culture
When researchers compared sulfur mustard to mechlorethamine (the original nitrogen mustard drug) in lab studies on airway cells, the two chemicals showed different killing profiles. Mechlorethamine was more potent at lower concentrations over a 24-hour exposure, while sulfur mustard killed cells faster during brief, high-dose exposures. These differences matter because cancer treatment depends on delivering a drug that kills tumor cells during the window of exposure while giving healthy tissue a chance to recover. The nitrogen mustard’s behavior made it more adaptable to controlled dosing schedules.
That said, nitrogen mustards are genuinely dangerous compounds. They are themselves carcinogenic, capable of causing the very type of DNA damage that leads to cancer.5PubMed. DNA damage and mutagenesis induced by nitrogen mustards This is the fundamental paradox of alkylating-agent chemotherapy: the same mechanism that destroys a tumor can, in a small number of patients, plant the seeds for a new cancer years later.
Nitrogen Mustard Drugs Still in Use
The original nitrogen mustard, mechlorethamine, is the most direct descendant of wartime research and is still used clinically. It has been part of the MOPP regimen (mechlorethamine, vincristine, procarbazine, and prednisone) for treating Hodgkin lymphoma and was tested in a randomized trial against another drug, vinblastine, in 172 patients with various lymphomas.6JAMA Internal Medicine. Chemotherapy of Lymphoma With Mechlorethamine and Vinblastine In pediatric oncology, the MOPP combination showed improved survival for children with medulloblastoma when added to radiation therapy, with five-year overall survival reaching about 74% compared to 56% for radiation alone.7Journal of Neurosurgery. Nitrogen mustard, vincristine, procarbazine, and prednisone as adjuvant chemotherapy in the treatment of medulloblastoma
Beyond intravenous use, mechlorethamine has a more surprising application: it is available as a topical gel for treating a type of skin lymphoma called mycosis fungoides. The FDA approved a mechlorethamine gel formulation in 2013 for early-stage disease, making it one of the rare cases where a chemotherapy drug is applied directly to the skin. It has been used in this way for over 60 years, though skin reactions can limit its tolerability.8PubMed Central. Management of Mycosis Fungoides with Topical Chlormethine/Mechlorethamine Gel
Cyclophosphamide is probably the most widely used nitrogen mustard derivative in modern oncology. Unlike mechlorethamine, it is a prodrug, meaning it is inactive when swallowed or injected and must be converted into its active form by liver enzymes, primarily one called CYP2B6.9PubMed Central. Cyclophosphamide Pharmacogenomic Variation in Cancer Treatment and Its Effect on Bioactivation and Pharmacokinetics This prodrug design gives it a broader therapeutic window: because the active compound is generated inside the body rather than being injected directly, it causes less immediate damage to the veins and tissues at the injection site. Cyclophosphamide is a key ingredient in the CHOP regimen (cyclophosphamide, doxorubicin, vincristine, and prednisone), which has cured roughly 30% of patients with advanced intermediate- or high-grade non-Hodgkin lymphoma.10PubMed. Comparison of a standard regimen (CHOP) with three intensive chemotherapy regimens for advanced non-Hodgkin’s lymphoma
Melphalan, another nitrogen mustard, takes a different approach to targeting. It is built on the backbone of the amino acid phenylalanine, which allows it to piggyback on the cell’s own amino acid transport systems to get inside. Research has shown that melphalan uptake is an active, carrier-mediated process involving at least two separate transport pathways, essentially fooling cells into importing it the way they would import nutrients.11Journal of Biological Chemistry. Active carrier-mediated transport of melphalan by two separate amino acid transport systems in LPC-1 plasmacytoma cells in vitro Melphalan is a mainstay of multiple myeloma treatment. High-dose melphalan at 200 mg/m² has been the standard conditioning regimen before autologous stem cell transplantation since 1992.12PubMed. High-dose BCNU/Melphalan conditioning regimen before autologous stem cell transplantation in newly diagnosed multiple myeloma
Other nitrogen mustard family members include chlorambucil, used for chronic lymphocytic leukemia and some autoimmune conditions, and bendamustine, a newer hybrid that combines the nitrogen mustard structure with a purine-like component. Nor-nitrogen mustard, yet another variant, is manufactured in large quantities as a chemical building block for producing multiple cancer drugs, though handling it requires stringent safety measures because it is mutagenic.13PubMed. Occupational exposure to nor-nitrogen mustard: chemical and biological monitoring
The Risk of Causing New Cancers
The irony of using DNA-damaging compounds to fight cancer is that they can cause cancer too. Alkylating agents as a class carry a well-documented risk of secondary malignancies, particularly leukemia. A study of childhood cancer survivors found that treatment with alkylating agents was associated with a roughly fivefold increase in leukemia risk, and in patients who received the highest cumulative doses, the risk climbed to about 23 times that of untreated children.14Journal of the National Cancer Institute. Leukemia After Therapy With Alkylating Agents for Childhood Cancer
Mechlorethamine appears to be one of the more problematic drugs in this regard. When researchers studied childhood Hodgkin disease patients treated with chemotherapy regimens that excluded mechlorethamine, the rate of secondary leukemia was lower than in studies that used the traditional MOPP regimen containing it. The difference was attributed to the lower cumulative alkylating agent doses and the specific absence of mechlorethamine.15PubMed. Low risk of secondary leukemias after chemotherapy without mechlorethamine in childhood Hodgkin’s disease This finding helped push oncology away from mechlorethamine-based regimens in many settings and toward alternatives like cyclophosphamide-based protocols, which still carry secondary cancer risks but are considered more manageable overall.
The secondary cancer risk is one of the main reasons oncologists carefully weigh cumulative alkylating agent exposure over a patient’s lifetime. If you’ve already received one course of alkylating chemotherapy and your cancer relapses, your oncologist will consider how much of these drugs you’ve already had before prescribing more.
How Tumors Develop Resistance to Nitrogen Mustards
One of the frustrations of nitrogen mustard therapy is that tumors can become resistant. The most thoroughly studied resistance mechanism involves a molecule called glutathione, a natural detoxifying compound found inside cells. Cancer cells that survive initial exposure to nitrogen mustards sometimes ramp up their glutathione production, essentially building an internal chemical shield. Resistant mammary carcinoma cells, for example, showed a twofold increase in glutathione levels and about a fivefold increase in the activity of the enzyme that uses glutathione to neutralize the drug.16PubMed. Nitrogen mustard-DNA interaction in melphalan-resistant mammary carcinoma cells with elevated intracellular glutathione and glutathione-S-transferase activity When researchers depleted this glutathione, the resistant cells became sensitive to the drug again, confirming the connection.
The resistance can be dramatic. One study of a nitrogen mustard-resistant cell line found roughly 15-fold resistance compared to the parent cells, and this resistance tracked with elevated levels of a specific detoxifying enzyme called glutathione S-transferase.17PubMed. Glutathione S-transferases in nitrogen mustard-resistant and -sensitive cell lines In the most extreme case studied, the resistant cells had amplified the genes encoding these protective enzymes by four to eight times and boosted production of the detoxifying protein itself by more than 40-fold.18PubMed Central. Amplification and increased expression of alpha class glutathione S-transferase-encoding genes associated with resistance to nitrogen mustards
Understanding these resistance pathways has practical consequences. Researchers have explored combining nitrogen mustards with agents that block glutathione production or inhibit the transferase enzymes, aiming to resensitize resistant tumors. This remains an active area of research, though no glutathione-blocking combination has become a standard protocol.
How Drug Size Affects Where DNA Gets Hit
Another layer of complexity involves how these drugs interact with DNA inside the cell nucleus. DNA is not a naked strand floating freely; it is wound tightly around protein structures called nucleosomes, and how tightly a particular region is wound affects whether a drug can reach it. Research using defined nucleosome structures found that most nitrogen mustard compounds preferentially damaged DNA in the “linker” regions between nucleosomes, where the strand is more exposed. Larger drug molecules had a harder time penetrating the tightly wrapped nucleosome core regions, while smaller molecules could access DNA even within those protected zones.19Chemical Biology & Drug Design. The influence of chromatin structure on DNA damage induced by nitrogen mustard and cisplatin analogues
This matters because genes that are actively being used by a cancer cell tend to have more open, accessible DNA, while genes that are silenced may be tightly packed. The drug’s size influences which genes it is most likely to damage, which in turn affects how lethal it is to a given tumor type and which side effects it causes in healthy tissue. It also explains, in part, why different nitrogen mustard drugs can have different clinical profiles despite sharing the same basic mechanism.
Designing New Variants
Chemists have continued modifying the basic nitrogen mustard structure to address specific clinical problems. One area of focus has been getting these drugs past the blood-brain barrier, the tightly sealed layer of cells that prevents most chemicals in the bloodstream from entering the brain. Standard nitrogen mustards are largely excluded by this barrier, which limits their usefulness against brain tumors. Researchers have synthesized nitrogen mustard agents linked to nicotinic acid carriers, creating compounds that are solid at room temperature and stable for over six weeks when stored at freezing temperatures, a practical advantage for hospital pharmacies.20PubMed. Synthesis and alkylation activity of a nitrogen mustard agent to penetrate the blood-brain barrier Getting an effective nitrogen mustard into brain tissue remains a difficult engineering problem, but the approach illustrates how the basic wartime chemistry keeps being repackaged for new therapeutic targets.
Nitrogen Mustards in Veterinary Oncology
The same drugs show up in veterinary medicine. Cyclophosphamide has been tested in dogs with lymphoma, where a randomized study found that adding it to a doxorubicin protocol did not significantly change complete response rates (about 73% versus 76%) or overall survival times compared to doxorubicin with a placebo.21PubMed Central. Doxorubicin and cyclophosphamide for the treatment of canine lymphoma: a randomized, placebo-controlled study The result was surprising and suggested that cyclophosphamide’s contribution may be less critical in canine lymphoma than in some human protocols.
Melphalan has also been evaluated as a single-agent rescue therapy for dogs whose lymphoma relapsed after other treatments. In a retrospective review of 19 dogs, oral melphalan produced a clinical benefit (partial response or stable disease) in about a third of cases, though the responses were short-lived, lasting roughly two to five weeks in responders. The drug was generally well tolerated, with blood-count changes being the most common side effect and gut toxicity being rare.22PubMed. Oral melphalan for the treatment of relapsed canine lymphoma For dogs that have run out of other options, oral melphalan’s low cost and ease of administration make it a practical if modest last-line treatment. The parallels with human oncology are striking: the same drugs, the same resistance problems, and the same trade-offs between efficacy and toxicity play out across species.