From Warfare to Mainstay: Mustard Derivatives Play Evolving Role in Cancer Therapy

An American soldier demonstrates gas masks for man and horse, circa 1918. Chemotherapy pioneer Jonathan L. Hartwell, PhD, right, at the National Cancer Institute about 1950. Photos courtesy of the National Archives, Bureau of Medicine and Surgery, and the National Cancer Institute

Mustard gas first rained down on sleeping British soldiers in July 1917. They awoke to the name-inspiring smell and the horrifying sensation that they were burning alive.

Several hundred men endured minutes of unspeakable pain before suffocating. They were the lucky ones. Many survivors endured hours of agony and forever lost their eyesight, along with much of their skin and most of their lung function.

By the 1918 armistice, mustard gas had killed or maimed tens of thousands and proved itself so effective a chemical weapon that Saddam Hussein chose it 7 decades later to slaughter Kurds.

So how was this ingredient in deadly chemical warfare transformed into the first chemotherapy agent in cancer treatment? As the oncology community takes note of the 40th anniversary of the “War on Cancer,” the leap from mustard gas to nitrogen mustard marks an intriguing touchstone in the battle against the disease and a case study of an oncologic agent that is still evolving.

An Accidental Discovery

The discovery of nitrogen mustard’s potential in cancer therapy could easily have taken place in 1919. Edward Bell Krumbhaar, MD, PhD, who would go on to become a leading pathologist and cardiac physician in Philadelphia, Pennsylvania, was a medical officer with the American forces in France when he studied the effects of mustard gas on soldiers and noted its tendency to kill bone marrow and suppress white blood cell production.¹

The breakthrough realization about the potential for using nitrogen mustard in cancer treatment, however, did not come until World War II, when the US government asked researchers at Yale School of Medicine in New Haven, Connecticut, to study potential antidotes to mustard gas as a weapon. They realized the agent’s promise as a treatment for lymphoid malignancies and began developing a mouse model for testing.²

The chemical first proved itself by sending scores of mice into miraculous, though temporary, remissions. Then in 1942, a 10-dose treatment of nitrogen mustard did the same for a 48-year-old man with lymphosarcoma.

The full story of that patient’s experience was not revealed until last year when 2 Yale School of Medicine physicians dug out his case history from a storage facility.² They learned that “JD,” a Polish immigrant who worked in a ball bearing factory, agreed to try the experimental treatment after radiation and surgery had failed to beat back the cancer. His treatment with a compound identified only as “a lymphocidal” or “substance X” because of wartime secrecy, spanned 96 days before his death.

Tests in other servicemen progressed equally well. The remissions were all temporary, but they were the first ever created by chemicals. It was a massive breakthrough, first published after wartime secrecy laws expired in 1946.³

In the years that followed, researchers and physicians bombarded nearly every tumor type with nitrogen mustard derivatives. Mustard compounds found a place in many of the chemotherapy cocktails that became the standard of care for most types of metastasized cancers.

Andrew Pearson, MD

Derivatives Studied in New Regimens

The compound’s potent lethality has ensured its descendents a place in oncology treatment regimens today, even amid advances in targeted therapies.

“We keep coaxing wonderful new tricks out of some very old dogs,” said Andrew Pearson, MD, a professor of Paediatric Oncology at the Institute of Cancer Research in London, in an interview.

Pearson led the clinical trials that established a regimen of busulphan and melphalan, a mustard gas derivative that is a half-century old, as the standard of care for high-risk pediatric patients with neuroblastoma.

Until 1987, no treatment showed any effect, and the disease, if metastasized, carried a swift death sentence. The prospects of long-term survival remain poor, with <40% of highrisk patients surviving beyond 5 years.

Fresh hopes for this patient population have emerged in recent years, however, through investigation of new ways to use melphalan, according to research presented at American Society of Clinical Oncology annual meeting in Chicago, Illinois, in June.⁴

The trial of the European SIOP Neuroblastoma Group confirmed that melphalan in combination myeloablative therapy with busulphan (BuMel) was superior to melphalan with carboplatin and etopsid (CEM) in 1577 high-risk patients. At 3 years, the BuMel combination demonstrated event-free survival for 49% of patients and overall survival in 60% of patients. (Busulphan, a methanesulfonate compound, is known in the United States as busulfan [Busulfex, Myleran].)

“

We can probably get much more out of melphalan and other mustard derivatives. ”

—Andrew Pearson, MD

The researchers found that the toxicity profile for BuMel was lower, despite a total veno-occlusive disease incidence of 18%, and recommended the regimen as standard treatment.

“We can probably get much more out of melphalan and other mustard derivatives,” said Pearson, who was a member of the research team on that study. “We don’t yet really understand how they work, not in detail at the DNA level, so we don’t really know how to use them best.”

Bendamustine Emerges

The nitrogen mustard compounds are part of the broader chemotherapy class of alkylating agents, which disrupt cancer cells by inhibiting the replication of DNA, a major process in tumor growth and survival. These compounds are able to form linkages with the nucleophilic centers of macromolecules, resulting in high, although nonspecific, chemical activity.⁵

Most of the mustard derivatives used to treat patients were developed from the late 1940s through the 1960s by replacing the sulfur in the mustard gas formula with nitrogen. In 2008, the class evolved further with the FDA approval of bendamustine hydrochloride (Treanda).

Bendamustine is structurally distinct from the other nitrogen mustard derivatives and includes both alkylating and antimetabolite properties with an acceptable toxicity profile, according to Leoni et al.⁶ The compound consists of an alkylating group, a benzimidazole ring, and a butyric acid side chain (Figure on Last Page).

As a result, bendamustine has a unique mechanism of action that results in the inhibition of several pathways, perhaps attacking not only DNA replication but also thwarting tumor cells that have developed resistance to apoptosis (cell death). The most frequent adverse events, low blood counts and an increase in bilirubin levels, are manageable and reversible after therapy is completed.⁷

Thomas E. Witzig, MD

Nevertheless, alkylating agents as a whole have been replaced in many treatment regimens during the past 20 years, a development that pleases Thomas E. Witzig, MD, hematologic oncologist and professor of Medicine at the Mayo Clinic in Rochester, Minnesota.

“Nitrogen mustard and all related chemicals have significant side effects,” Witzig said in an interview. “When used to treat Hodgkin’s in the 1970s and ’80s, it tended to cause nausea and was associated with infertility. It was effective at curing Hodgkin disease but later in life these patients had an increased risk of second cancers. So, we were happy to see it replaced in the late 1980s by other regimens.”

As a young doctor, Witzig participated in the comparative trials that replaced the MOPP (Mustargen/ Oncovin/procarbazine/prednisone) cocktail featuring the mustard derivative with mustard-free ABVD (Adriamycin/bleomycin/vinblastine/dacarbazine) as the standard of care for Hodgkin lymphoma. He is continuing research aimed at replacing elements of ABVD with still gentler substances.

Yet Witzig has found situations that demand the return to the former standard of nitrogen mustard.

“Plain old nitrogen mustard is the only thing that will work in people whose lymphoma has caused liver failure. Unlike any other chemical, it can fight cancer the instant it hits the body rather than requiring that the liver first transform it into an active agent,” said Witzig, who switches back to other treatments once the nitrogen mustard has worked enough to restore liver function.

“I don’t need it often,” Witzig explained, “but there are times when it can save lives that no other drug can.”

Chemical Compositions of 4 Alkylating Agents

The chemical structure of bendamustine, top, differs significantly from nitrogen mustard derivatives developed in earlier eras.⁶

Drug Shortages Can Occur

Pure nitrogen mustard has fallen so far from favor that only limited quantities are manufactured and distributed, said Witzig, who noted that the pharmacy department at Mayo Clinic has been unable to buy any recently, so it now barters with other hospitals to obtain his supplies.

Indeed, Lundbeck Inc, the only company that supplies mechlorethamine hydrochloride for injection (Mustargen) to oncologists in the United States, has been trying to resolve manufacturing difficulties for several years. More than 20 contractors declined offers to manufacture the product, including a contractor who bowed out after Lundbeck invested more than $3 million in a new facility, Lundbeck said in a letter to healthcare providers in April 2010.⁸

In October, the American Society of Health- System Pharmacists said shipments of 10-mg vials of Mustargen are expected to resume during the second quarter of 2012. The group advised prescribers to shift patients to alternative regimens, taking into account renal function, liver function, and neoplasm type and location.⁹

Other mustard derivatives, with the occasional exception of melphalan, are available because, while they have been selectively displaced by other drugs, they still remain in heavy rotation. For example, cyclophosphamide forms part of nearly all of the common chemotherapy cocktails used to fight breast cancer. Melphalan, likewise, remains the standard of care for multiple myeloma in much of the world.

Other mustard derivatives, while not the first medications used, still merit second- or third-line treatment status against ovarian, colorectal, and testicular cancer, as well as malignant melanoma and some lymphomas and leukemias.

Indeed, the FDA expanded its approval for Treanda in 2009 to include patients with chronic lymphocytic leukemia and indolent B-cell non- Hodgkin lymphoma whose disease has progressed during or within 6 months of prior treatment, and it is being tested for use against sarcoma.

The Unique Qualities of Mustard Derivatives

“The biggest problem with the mustard agents is also their greatest strength: They attack all kinds of cells,” said Dale Shepard, MD, PhD, an oncologist and researcher at the Cleveland Clinic in Ohio.

All mustard agents, from the original weapon sulphur mustard, to its medical cousin nitrogen mustard, to all of the later variations, invade human cells indiscriminately and attach themselves to DNA within the cells. They nearly always prevent cells from reproducing. Often, they kill the cells as well. Since cancer cells typically reproduce exponentially faster than normal cells, mustard agents have proved particularly harmful to their normal progression.

Dale Shepard, MD, PhD

“Targeted therapies do absolutely nothing for tumors that lack whatever characteristic they target,” Shepard said. “Even gentler types of traditional chemotherapy, medications that have become the standard of care, can fail against particularly robust tumors. Sometimes, you need to use the hammer.”

The key, Shepard said, is using “the hammer” as little as possible, which is why he is happy to report that these days he rarely uses anything but targeted therapies against kidney cancers and many of the other tumor types he treats.

In other areas, however, he still has to use alkylating agents, although he notes that even in cases where he must use such “hammers,” he can swing them more gently than he once did. “We still use a lot of the old drugs, but we use them differently. We understand more about timing and dosage so we can, in many cases, maintain treatment efficacy while reducing side effects.”

Searching for New Therapeutic Uses

Researchers are also looking for ways to use those traditional drugs against even more targets. Regarding melphalan, for example, the FDA reports that clinical trials are underway to test the drug for new therapeutic uses against nearly a dozen tumor types: breast cancer, colorectal cancer, Ewing’s sarcoma, hematologic cancer, leukemia, liver cancer, lymphoma, multiple myeloma, neuroblastoma, ovarian epithelial cancer, and primary systemic amyloidosis. A search of ClinicalTrials.gov yielded nearly 180 active investigations that involve melphalan.

At the same time, researchers are seeking biomarkers that will help target patients who would benefit from melphalan and other alkylating agents.

Scientists at the Aarhus University Hospital in Aalborg, Denmark, used microarray-based gene expression profiles to analyze B-cell lines in vitro to develop a melphalan resistance index in patients with multiple myeloma.¹⁰ The index must be validated and correlated with other biomarkers, they said.

The need for a predictive tool is pressing. Although melphalan is the “backbone” of current multiple myeloma therapy, with highdose treatment followed by autologous stem cell transplantation, “overall survival remains dismal and the disease is considered incurable—mainly due to an initial refractory disease or induced resistance resulting in disease relapse,” the researchers said in their paper.

With all of that work underway—and much of it showing promising results—oncology researchers expect that mustard derivatives will be in use for decades to come. And a substance developed to be one of the worst poisons in history ultimately will be remembered as the source of one of the greatest therapies.

The Nitrogen Mustard Derivatives Family

The first chemotherapy drugs were developed by replacing the sulfur in the chemical weapon mustard gas with nitrogen. Today, there are 6 antineoplastic agents derived at least in part from nitrogen mustard. Researchers are exploring a variety of new ways to use most of these agents, as demonstrated by the clinical trials noted in this table. One agent, mechlorethamine (Mustargen), is rarely used and difficult to obtain.

Drug^a

Oncologic Uses

FDA Approval

Selected Clinical Trials

Bendamustine

(Treanda)

Chronic lymphocytic leukemia

Indolent B-cell non-Hodgkin lymphoma progressed within 6 months of treatment with rituximab alone or in regimen

2008

Lenalidomide, bendamustine, and ritixumab as first-line therapy for patients over 65 with mantle cell lymphoma (NCT00963534)

A study to investigate the efficacy and safety of bendamustine compared with bendamustine RO5072759 (GA101) in patients with rituximab-refractory, indolent non-Hodgkin's lymphoma (GADOLIN) (NCT01059630)

Significance of duration of maintenance therapy with rituximab in non-Hodgkin lymphomas (MAINTAIN) (NCT00877214)

Chlorambucil

(Leukeran)

Chronic lymphocytic leukemia

Hodgkin lymphoma

Non-Hodgkin lymphoma

1957

Study of the effectiveness and safety of lenalidomide versus chlorambucil as fi rst line therapy for elderly patients with B-cell CLL (The ORIGIN Trial) (NCT00910910)

A study to compare MabThera (rituximab), fludarabine and cyclophosphamide to Mabthera and chlorambucil in patients with chronic lymphocytic leukemia and unfavourable somatic status (NCT01283386)

CLL11: A study of RO5072759 with chlorambucil in patients with previously untreated chronic lymphocytic leukemia (NCT01010061)

Cyclophosphamide

(Cytoxan)

Leukemias, including acute lymphoblastic leukemia, acute myeloid leukemia, chronic lymphocytic leukemia, chronic myelogenous leukemia

Hodgkin lymphoma, non-Hodgkin lymphoma

Breast cancer

Multiple myeloma

Neuroblastoma

Ovarian cancer

Retinoblastoma

1959

A clinical trial comparing the combination of TC plus bevacizumab to TC alone and to TAC for women with node-positive or high-risk node-negative, HER2-negative breast cancer (NCT00887536)

IMA901 in patients receiving sunitinib for advanced/metastatic renal cell carcinoma (NCT01265901)

Intercontinental multidisciplinary registry and treatment optimization study for choroid plexus tumors (NCT01014767)

Ifosfamide

(Ifex)

Germ cell testicular cancer

Bone and soft-tissue sarcomas

Lung cancer

Cervical cancer

Ovarian cancer

1988

Paclitaxel and carboplatin or ifosfamide in treating patients with newly diagnosed persistent or recurrent uterine or ovarian cancer (NCT00954174)

Panobinostat plus ifosfamide, carboplatin, and etoposide (ICE) compared with ICE for relapsed Hodgkin lymphoma (NCT01169636)

PET scan combined with CT scan in predicting response in patients undergoing chemotherapy and surgery for soft tissue sarcoma (NCT00346125)

Mechlorethamine

(Mustargen)

Bronchogenic carcinoma

Chronic lymphocytic, chronic myelogenous leukemia

Hodgkin, non-Hodgkin lymphoma

Melanoma

Breast cancer

Renal cell, gastrointestinal carcinomas

1949

^bLundbeck Inc, the only supplier of Mustargen in the United States, announced an extended back order for the drug in April 2010 due to difficulties in contracting a manufacturer.

Prescribers are urged to transition patients to other regimens based on renal function, liver function, and the neoplasm type and location.

Melphalan

(Alkeran)

Multiple myeloma

Ovarian cancer

Breast cancer

Chronic myelogenous leukemia

Osteogenic sarcoma

Advanced prostatic carcinoma

Testicular seminoma

1964

Addition of ipilimumab (MDX-010) to isolated limb infusion (ILI) with standard melphalan and dactinomycin in the treatment of advanced unresectable melanoma of the extremity (NCT01323517)

Bevacizumab plus gemcitabine, docetaxel, melphalan, and carboplatin in ovarian cancer patients (NCT00583622)

Melphalan and dexamethasone with or without bortezomib in treating patients with previously untreated systemic light-chain amyloidosis (NCT01078454)

^aChemical illustrations courtesy of the National Center for Biotechnology Information. PubChem Database.

^bAmerican Society of Health-System Pharmacists. Mechlorethamine hydrocholoride injection. www.ashp.org/DrugShortages/Current/bulletin.aspx?id=25#. Published October 5, 2011. Accessed October 31, 2011.

Sources: NCI Drug Dictionary, Drugs@FDA Website, MedlinePlus, ClinicalTrials.gov, prescribing information for individual drugs.

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