According to NCI, “Cancer is a term used for diseases in which abnormal cells divide without control and are able to invade other tissues” [318]. In fact, most definitions use “uncontrolled” proliferation or growth at their core. More generic terms include tumors and neoplasms, though they can be benign, pre-malignant, or malignant. Implicit in the terms tumor (abnormal mass) and neoplasm (new growth) is the notion that these processes, particularly in their malignant variety, like invading bacteria, are inherently different from the host and must be thoroughly eradicated in order to prevent metastases and death. The application of the infectious disease model to cancer steered cancer research, diagnosis, treatment, and outcome assessment strategies towards both surgical excision of early-stage disease and the cancer cell-killing paradigm to eradicate advanced cancer, which is the focus of this chapter. From this basis, two major practical corollaries followed. The first is that cancer research has been oriented towards the search for therapeutically exploitable differences between cancer and normal cells, guided by successive hypotheses ranging from excessive cancer cell proliferation [319], a misconceived generalization that drove drug use for decades, to tumor-specific antigens targetable for therapy [320], an illusion not yet abandoned. As decried in a recent article, “It could be argued that medical treatment of cancer for most of the past century was like trying to fix an automobile without any knowledge of the internal combustion engines or, for that matter, even the ability to look under the hood” [321]. The second corollary is the concept of “cytotoxicity” (e.g., cell killing) of rapidly dividing cells introduced to describe the quintessential property that drugs must exhibit in order to be successful in the treatment of disseminated cancer. However, how these drugs were to kill cancer cells preferentially while sparing normal cells was never adequately explored nor fully explained. The notion of cell-killing as the cornerstone of cancer treatment became untenable when the carcinogenic process was shown to involve oncogenes that promote cell growth, mutated tumor suppressor genes that fail to counteract cancer-promoting oncogenes, defective DNA repair genes that enable replication and propagation of unstable genomes, microRNA that control the expression of most human genes, or defective cell death pathways that confer a survival advantage to cancer cells. From this flawed concept about cancer treatment, an entire lexicon emerged in attempts to explain empirical clinical observations. For example, the tendency of some tumors to outgrow adjacent normal tissues, a phenomenon that can be slowed and sometimes stopped by anti-cancer drugs, suggested a pivotal role for the cell cycle in tumor growth and anti-cancer drug activity. Thus, cancer drugs were classified as cell cycle dependent if they acted upon one of the phases of the cell cycle, and cell cycle independent if their anti-tumor activity was independent of the cell cycle. The former, in turn, were classified as S-specific (drugs such as the antimetabolites and anti-purines that inhibit DNA synthesis), M-phase dependent (drugs that arrest mitosis, such as Vinca alkaloids, Podophyllotoxins and Taxanes), or G₁– and G₂-phase dependent, such as Corticosteroids and Asparaginase, and Bleomycin and Topotecan, respectively. Cell-cycle independent drugs included the alkylating agents, such as Busulfan, Melphalan, and Chlorambucil that, by crosslinking guanine nucleobases on the DNA, prevent uncoiling and replication of the double helix, hence the cell division. Mechanism of action to a large degree determined the type of toxicity. Likewise, it was quickly discovered that anti-tumor activity was dose-dependent, but, given its non-specificity, dose escalation was limited by type and severity of toxicity resulting from drug effect on normal cells. Thus, in order to enhance anti-tumor activity while reducing toxicity, drugs with different mechanisms of action were combined and administered intermittently to reduce toxicity on normal tissues, especially the high turnover bone marrow, and enable time to recover from toxicity between treatment cycles. Perhaps the most successful example of this approach was the MOPP (Nitrogen mustard, Vincristine, Prednisone, and Procarbazine) chemotherapy regimen for Hodgkin’s disease that proved curative in most cases [322]. However, this early success was seldom replicated despite a myriad of clinical trials launched to test a variety of intermittent combination chemotherapy regimens in many types of cancers over the ensuing four decades.

In response to the marginal success achieved by cytotoxic chemotherapy in the management of most advanced malignancies, cancer researchers explored new treatment modalities with renewed enthusiasm and unrealistic expectations. One such direction was based on the immune surveillance hypothesis that emerged from observations made in the 1960s of an increased cancer risk in patients with severe congenital or acquired immunodeficiencies [323]. According to this hypothesis, cancer cells emerge from time to time but are eliminated by a sort of search-and-destroy defense mechanism before they can develop into full-blown tumors. Defects in immune surveillance were believed not only to contribute to cancer development but also to prevent the elimination of the few cancer cells remaining after successful chemotherapy, thus precluding relapses. This conceptually attractive hypothesis found widespread following. For example, at the International Conference on Immune Surveillance held at Brook Lodge, MI in May 1970, the Chairman opened the meeting declaring,

Everyone here surely accepts the reality of tumor-specific immunity and would also favor the proposition that cell-mediated immune mechanisms have something to do with recognition and attack on tumor-specific antigens [324].

Proponents of the immune surveillance theory, supported by rare cases of “spontaneous” regressions of several human solid tumors [325], suggested that immune defects could be overcome and anti-tumor activity might be enhanced by immune stimulants. Experimental attempts to potentiate the anti-cancer properties of the immune system begun in the mid-1960s using BCG (Bacillus de Calmette Guérin), an attenuated strain of Mycobacterium bovis for the treatment of childhood leukemia [326], were followed in the 1970s and 1980s by the introduction of the Interferons [327], Levamisole [328], and the highly toxic Interleukins [329]. As the concept of cancer immunotherapy gathered momentum, new agents such as colony-stimulating factors and monoclonal antibodies [330] were added to the list under the evocative name Biological Response Modifiers, which are at the core of Biological Therapy, or Biotherapy for short. Their mechanism of action is said to “alter the interactions between the body’s immune defenses and cancer, to boost, direct, or restore the body’s ability to fight the disease” [331] and to,

Each immune enhancer rode a wave of enthusiasm in the medical community and in the press. For example, Interferon, discovered in the early 1950s by Nagano and Kojima [333] (but attributed to Isaacs and Lindenmann in the English-language literature [334]) and produced in large scale from human white blood cells in the 1970s [335] or from cultures of genetically modified bacteria in the 1980s [336], and later from yeast and from recombinant mammalian cells [337], was greeted with a deluge of global media coverage thanks to astute promoters. It was touted as a “magic bullet”, a “miracle cure”, or “the genie in a fairy tale” that was equally effective for curing the common cold and cancer. Business media touted Interferon as a “gold mine for patients and for companies.” General enthusiasm about Interferon led the American Cancer Society to award, in the late 1970s, a $2 million grant, the largest in its history, to conduct clinical trials, and biotechnology firms Burroughs-Wellcome, Hoffmann-La Roche, and Schering-Plough to allocate large portions of their research and development (R&D) budget to Interferon. However, 2 months after Time Magazine heralded on its cover “Interferon: The IF drug for cancer”, a May 1980 New York Times article raised doubts about the anti-cancer efficacy of Interferon based on unpublished clinical trial results. In response to the article, four scientists from the Sloan Kettering Institute for Cancer Research wrote a letter to the newspaper expressing concern that such reporting might undermine public support for interferon research. Eventually, as results of clinical trials became known, the public mood switched from premature enthusiasm to hasty pessimism, especially when four patients treated with interferon in France died as a result of the treatment. An historical analysis of the impact of the media on public perception of science [338] made the following observations using Interferon as an example. “First, imagery often replaced content…Second, the press covered Interferon research as a series of dramatic events. Readers were treated to hyperbole, to promotional coverage designed to raise their expectations and whet their interest.” The role of scientists was described as follows: “Far from being neutral sources of information, scientists themselves actively sought a favorable press, equating public interest with research support.” Nothing original here, for politicians most often claim their personal views to reflect their constituents’. Interleukin-2 (IL-2) was another darling of the media through the early 2000s, as typified by the premature enthusiasm of its main promoter, who wrote, “The demonstration that even bulky invasive tumours can undergo complete regression under appropriate immune stimulation by IL-2 has shown that it is indeed possible to treat cancer successfully by immune manipulation” [339], and his numerous guest appearances on ABC’s “World News Tonight with Peter Jennings”. Today, BCG is used for treating in situ bladder cancer, Interferons are active in Hairy cell leukemia, an extremely rare form of leukemia (fewer than 700 yearly cases in the US), and only marginally beneficial to 15 % of patients with disseminated skin melanoma and kidney cancer. Likewise, despite encouraging early reports [340] of IL-2 as part of a triad involving chemotherapy and “tumor-infiltrating” T-lymphocytes, its use is limited by severe adverse effects often requiring intensive care management when treating the only two types of cancer (e.g., skin melanoma and kidney cancer) for which it has shown relative efficacy. Indeed, the approximate 15 % long-term complete and partial responses in patients receiving high-dose IL-2 must be balanced by a concomitant 4 % death rate from complications [341].

A variant of immunotherapy was based on searching for antigens that could be used as therapeutic targets or for generating immune-enhancing vaccines. A dual strategy has been pursued: to attempt inducing antigen-specific immune responses in cancer patients or prevent its development. The former has used whole cancer cells or single-antigen peptides derived from cancer cells used alone or as complex cocktails combined with cytokines or other adjuvants¹ as an immune enhancer [342]. These approaches have been largely unsuccessful. Indeed, Provenge® was the first and thus far only FDA-approved (29 Apr 2010) therapeutic cancer vaccine. It is used for the treatment of hormone-refractory metastatic prostate cancer [343]. Although initially hailed as a breakthrough immunotherapy for prostate cancer, Provenge® has shown only a modest 4-month survival advantage compared to a placebo [344], despite an approximate $31,000 per infusion cost. In its second full year of marketing, Provenge® generated $325 million. In the meantime, another direction of the War on Cancer that generated enormous enthusiasm and consumed large resources was the virus link. The old hypothesis that viruses caused most cancers was revived with renewed interest following the 1981 discovery of HTLV-1,² the first retrovirus [345], and of HIV³ 2 years later [346]. Given the high stakes involved, the latter was marred by controversy and legal action [347], accentuated by the 1988 Nobel Prize in Physiology or Medicine awarded to two scientists of only one of two laboratories involved [348] that eventually ended in a sanitized version of the events written by the main participants [349]. The new push to find cancer-causing viruses, vigorously promoted and generously funded by NCI, helped establish a cancer link to eight viruses, listed in Table 7.1.

However, by the mid-1990s, it became clear that the notion that most cancers were caused by viruses was a false lead, and the idea was largely discarded. Nevertheless, a successful strategy has emerged to use vaccines against known cancer-causing or promoting viruses responsible for approximately 15 % of all cancers. To date, the FDA has approved Cervarix® (16 Oct 2009) and Gardasil® (22 Dec 2010), two highly efficacious cancer-preventive vaccines that protect against the HPV-16 and HPV-18 infections that cause approximately 70 % of all cases of cervical cancer worldwide. While Gardasil® and Cervarix® are highly effective in cervical cancer prevention, our inability to develop effective bio-therapeutic agents and the very modest survival outcome gain associated with the use of Provenge® suggest that therapeutic immunotherapy is unlikely to play a prominent role in the future management of most cancers. On the other hand, die-hard supporters of immunodeficiency as a cause of cancer extend the scope of what they consider immune enhancers to products with very different mechanisms of action, including nonspecific inflammatory inducers, cytokines, monoclonal antibodies, and immunotoxins [361]. Regardless, after decades of clinical trials, at great human and financial cost, therapeutic immunotherapy of any type has shown marginal usefulness in the adjuvant setting and essentially none as primary treatment of advanced disease [362]. This is not surprising given our current knowledge of cancer genetics and epigenetics, which suggests that most cancers develop and progress not as a result of immune deficiencies or by escaping immune detection, but driven by factors and mechanisms independent of any distinct cancer cell feature recognizable by the host’s immune system.

While the search for anti-cancer agents progressed at a snail’s pace, giant strides were being made in the development of both technologies and assays designed to detect internal and intra-cavitary tumors in early stages. The former include imaging techniques such as computerized axial tomography (CAT- or CT-scan), magnetic resonance imaging (MRI), and ultrasound, all suited to detect cancer at the multi-cellular level. The latter include cellular and molecular methods, such as cytogenetics, fluorescence in-situ hybridization, comparative genomic hybridization, spectral karyotype, microarrays, flow cytometry, genomic analysis, and polymerase chain reaction (PCR), all capable of detecting specific abnormalities at the subcellular or molecular levels [363, 364]. For example, PCR, a powerful molecular tool applicable to hematologic malignancies, enables detection of as few as one leukemia or lymphoma cell out of one million normal cells [365]. To these must be added more ordinary laboratory testing for cell products that, although produced by normal cells often are produced in excess by cancer cells, such as PSA and Chorioembrionic Antigen, which are associated with prostate and colon cancer, respectively. Such remarkable discriminant diagnostic power has thrust the definition and notion of complete remission from the clinical and pathologic domains to the molecular realm and to detection of increasing numbers of surgically resectable and potentially curable early stage cases. However, this has had two unintended consequences: one, fostering more aggressive and prolonged chemotherapy in attempts to eradicate the very last detectable cancer cell and achieve complete molecular remissions, inevitably resulting in greater toxicity; the other, in diagnosing and treating early non-progressive disease with little impact on the bearer’s survival. Regardless of its definition, complete remissions are rarely achieved and true cures remain elusive, forcing the coinage of an entire lexicon of terms designed to characterize and quantify intermediate treatment outcomes. These fall under two general categories: tumor outcomes and patient outcomes. The former includes an array of terms that often complicate direct comparison of clinical trials results, such as partial and complete remission, response duration, and time to progression. The latter is measured by survival prolongation and Quality of Life (QOL) or health-related QOL (HRQOL) to be further discussed in Chap. 9. Tumor outcome assessment is useful as an early indication of the effectiveness of a particular therapy, but not for predicting patient survival, despite the fact that meaningful survival prolongation is generally preceded by complete remissions. On the other hand, patient survival constitutes the gold standard for gauging the success or failure of cancer treatment, as advocated by ASCO’s Health Services Research Committee [366]. Survival rates are said to be relative when describing survival rates in cancer patients compared to those for persons in the general population matched for age, gender, race, and calendar year of observation. Relative survival also adjusts for life expectancy in the population at large. Unless qualified (such as disease-free or relapse-free), relative survival rates include persons who are living after diagnosis, whether disease-free or not. From a practical standpoint, 5-year survival is the preferred benchmark as a meaningful and achievable indicator of treatment outcome. Yet, while most patients achieve some degree of tumor response, 5-year survival rates remain unsatisfactory for most patients with advanced cancer, as discussed in Chap. 9. While all these terms were designed to assess, compare, and communicate outcomes of clinical cancer research, tumor response has become entrenched in the clinical setting as an indication of treatment success or failure. This is because, while patient survival is judged in retrospect, tumor response is attractive to both physicians and patients, for it allows assessment of tumor status at each step, including marking the first step towards a complete remission and, it is hoped, prolonged survival or a need to change direction. However, focusing on tumor responses rather than on patient survival is an implicit acknowledgment of the ineffectiveness of anti-cancer agents and of the unresponsiveness of most cancers to such agents, and detracts clinicians from their primary raison d’être, mainly designing management plans to optimize patient welfare rather than relying on mostly ineffective drugs in attempts to maximize tumor shrinkage at any cost. This latter approach also misleads patients, given the promises implied in evocative words such as response and remission that permeate Oncologist-patient interactions, as further discussed in Chap. 12. More on this under the section, The New Targeted Therapeutics at the end of this chapter.

While surgery is most adept and successful at managing early stage cancer, Medical Oncology is the discipline that uses FDA-approved agents for treating advanced, inoperable cancer. Today, the vast majority of patients with disseminated or metastatic cancer are treated with chemotherapeutic drugs either alone or with surgery, radiotherapy, or biological agents as adjuvants. Cancer chemotherapy is a recent development, with its historical origins in observations of the toxic effects of mustard gas (sulfur mustard) in WWI servicemen, in soldiers and civilians accidentally exposed during the Bari raid during WWII, and in animal and human experimental studies preceding and during WWII, respectively. Mustard gas is the common name for 1,1-thiobis(2-chloroethane), a vesicant chemical warfare agent synthesized in 1860 by Frederick Guthrie (1833–1886) [367] and first used on July 12, 1917 near Ypres (Flanders). Thus, its alternate name: Yperite. Because it could penetrate masks and other protective equipment available during WWI, and given its widespread use by both sides of the conflict, its effects were particularly horrific and deadly. Out of 1,205,655 soldiers and civilians exposed to Mustard gas during WWI, 91,198 died [368]. In 1919, a captain in the US Medical Corps reported decreased white blood cell counts and depletion of the bone marrow and of lymphoid tissues in survivors of mustard gas exposure he treated in France [369]. Shortly thereafter, military researchers from the US Chemical Warfare Service reported similar effects in rabbits injected intravenously with dichloroethylsulfide contaminated with mustard gas [370]. Other reports between 1919 and 1921 described various properties of dichloroethylsulfide in vitro and in laboratory animals [371–373], previously developed for screening thousands of potential anti-cancer compounds [374, 375]. Fifteen years later, the anti-cancer activity of mustard gas in experimental animal models was reported for the first time [376]. At the beginning of WWII, the Office of Scientific Research and Development (OSRD), an agency of the US War Department, funded Milton Winternitz of Yale University to conduct secret chemical warfare research in search of antidotes [377]. Winternitz asked Alfred Gilman and Louis Goodman to assess these agents’ therapeutic potential. Their initial studies confirmed the toxicity of nitrogen mustard (where the sulfur atom on the mustard gas is substituted by a nitrogen atom) on rabbits’ blood cells, and later documented anti-tumor activity in mice xenotransplanted with a lymphoid tumor. These encouraging results led to the first experimental use of nitrogen mustard on JD, a 48 year-old Polish immigrant with refractory lymphosarcoma. Given the secrecy surrounding mustard gas studies, which remained in place well after the war had ended, JD’s records were lost until May 2010, when two Yale surgeons found their off-site location through persistence and luck and revealed their content at the a Yale Bicentennial Lecture on January 19, 2011 [378]. Unsurprisingly, nowhere in JD’s record was nitrogen mustard mentioned, with references instead to a “lymphocidal” agent or “substance X.” Given its historical significance and interest to our readers, a synopsis of JD’s clinical case is included.

In August 1940, JD developed rapidly enlarging tonsillar, submandibular, and neck lymph nodes that revealed lymphosarcoma when biopsied. Referred to Yale Medical Center in February 1941,

He underwent external beam radiation for 16 consecutive days with considerable reduction in tumor size and amelioration of his symptoms. However, his improvement was short lived, and by June 1941, he required additional surgery to remove cervical tumors. He underwent several more cycles of radiation to reduce the size of the tumors, but by the end of the year they became unresponsive and had spread to the axilla. By August 1942, two years after the initial onset of symptoms, he suffered from respiratory distress, dysphagia, and weight loss, and his prognosis appeared hopeless [379].

Having exhausted standard lymphoma treatment, Drs. Gillman, Goodman, and Gustaf Lindskog, a Yale surgeon, offered JD Nitrogen Mustard as an experimental treatment.

At 10 a.m. on August 27, 1942, JD received his first dose of chemotherapy recorded as 0.1 mg/kg of synthetic lymphocidal chemical. This dosage was based on toxicology studies performed in rabbits. He received 10 daily intravenous injections, with symptomatic improvement noted after the fifth treatment. Biopsy following completion of the treatment course remarkably revealed no tumor tissue, and he was able to eat and move his head without difficulty. However, by the following week, his white blood cell count and platelet count began to decrease, resulting in gingival bleeding and requiring blood transfusions. One week later, he was noted to have considerable sputum production with recurrence of petechiae,⁴ necessitating an additional transfusion. By day 49, his tumors had recurred, and chemotherapy was resumed with a 3-day course of “lymphocidin”. The response was short-lived, and he was administered another 6-day course of substance “X”. Unfortunately, he began experiencing intraoral bleeding and multiple peripheral hematomas and died peacefully on December 1, 1942 (day 96). Autopsy revealed erosion and hemorrhage of the buccal mucosa, emaciation, and extreme aplasia of the bone marrow with replacement by fat [380].

Given the secrecy surrounding research involving war gas, all experimental studies were kept secret until 1946 when the Yale researchers were allowed to begin publishing their wartime clinical experiments, the first of which included the following disclosure:

This paper was prepared as a background for forthcoming articles on the clinical application of the 3-chloroethyl amines with the approval of the following agencies: Medical Division, Chemical Warfare Service, United States Army; Division 9, NDRC, and Division 5, Committee on Medical Research, OSRD; Committee on Treatment of Gas Casualties, Division of Medical Sciences, NRC; and Chemical Warfare Representative, British Commonwealth Scientific Office [381].

In the meantime, mustard gas was brought to the medical community’s attention by a WWII incident when servicemen and civilians were accidentally exposed to the agent released during the bombardment of the Italian town of Bari by Hitler’s Luftwaffe, on December 2, 1943, launching the era of cancer chemotherapy [382].

Bari was a usually sleepy town of approximately 65,000 people, capital city of the Apulia region on the Adriatic coast of the Italian “boot”. Old Bari, perched on a promontory around its medieval fortified Castello Normanno Svevo, built in 1132 by Norman King Roger II, and the Basilica di San Nicola, built between 1087 and 1187, along with new Bari were transformed in late 1943 by the arrival of approximately 30 allied ships in its small harbor. Under British jurisdiction, Bari was the main supply center for British General Montgomery’s Army, and had just been designated headquarters of the American Fifteenth Air Force division. Occasionally, German reconnaissance planes would fly over Bari undisturbed by the Allies who believed the Luftwaffe was spread too thin to mount a successful attack on the city. In the early afternoon of December 2, 1943, Werner Hahn, flying his Messerschmitt Me-210 reconnaissance plane, made two undisturbed high altitude passes over the city, reporting to his headquarters, led by Marshal’s Albert Kesselring and Wolfram von Richthofen, two of the best and most underrated German tacticians of the war, the suitability of Bari as a target for an air strike. Later that day, British Air Vice-Marshall Sir Arthur Conningham held a press conference. Answering war correspondents’ pointed questions regarding lax security, he declared, with characteristic British self-confidence, “I would consider it a personal insult if the enemy should send so much as one plane over the city” [383], despite knowing the town had no meaningful air or port defenses. A few hours later, a squadron of 105 twin-engine Junkers Ju-88 A-4 bombers led by Lieutenant Gustav Teuber left their base in northern Italy and, flying low to evade Allied radar, descended on Bari in a surprise air raid that would become known as the “second Pearl Harbor”. When the squadron arrived, the German pilots could hardly believe their eyes and their luck: The entire harbor was brightly lit, highlighting ships and personnel unloading cargo! A few rounds fired by the sole, antiquated anti-aircraft battery in the city were futile. By 19:50, 20 min after the raid began, 28 merchant ships and 8 allied ships were sunk or destroyed, including the U.S.S. John Harvey, a 7,176-ton Liberty-type American ship, carrying a secret load of 2,000 M47A1 60–70 lb sulfur mustard (mustard gas) bombs [384, 385]. Some of the mustard bombs were damaged,

…causing liquid mustard to spill out into water already heavily contaminated with an oily slick from other damaged ships. Men who abandoned their ships for the safety of the water became covered with this oily mixture that provided an ideal solvent for sulfur mustard. The casualties were pulled from the water and sent to medical facilities unaware of what they carried with them on their clothes and skin. Equally unaware were the medical personnel who treated these casualties. Before a day passed, symptoms of mustard poisoning appeared in both the casualties and the medics. This disturbing and puzzling development was further compounded by the arrival of hundreds of civilians for treatment; they had been poisoned by a cloud of sulfur mustard vapor that blew over the city from some of the bombs that had exploded when the ship sank [386].

A witness of that evening’s pandemonium later reported,

Some mustard gas sank to the bottom of the harbour, but a lot floated on the oil. Many of the survivors – as well as rescuers, some of whom dived into the water to rescue others – were covered in mustard gas. The gas, oil, and phosphorus caused frightening burns. Also when the men reached the hospital in Bari, the heat in the operating theatres evaporated the mustard gas, allowing it to get into the surgeon’s eyes, creating dreadful results [387].

At first, casualties seemed relatively modest compared to the extent of materiel losses. However, the symptoms exhibited by many survivors were not usually seen among war casualties. Moreover,

The destroyer U.S.S. Bistera, well outside the harbor and undamaged by the raid, had pulled 30 men from the water in a rescue effort. By the next day, the officers and crew of the Bistera were blinded from the effects of the sulfur mustard carried onto the ship by those rescued [388].

Informed of the mysterious malady, Deputy Surgeon General Fred Blesse dispatched Lt. Col. Stewart Francis Alexander, a military physician. From his experience treating mustard gas victims during WWI, Dr. Alexander quickly suspected mustard gas. Carefully tallying the location of the victims at the time of the attack, he was able to trace the epicenter to the John Harvey, confirming mustard gas as the culprit when he located a fragment of an M47A1 bomb, which he knew carried the agent. By the end of the month, 83 of the 628 hospitalized military mustard gas victims had died. The number of civilian casualties, thought to have been much greater, could not be ascertained accurately, because most had sought refuge with relatives out of town. This would be the only episode of exposure to a chemical warfare agent during WWII. Allied Supreme Commander General Dwight D. Eisenhower approved Dr. Alexander’s full report, though details of the episode were not declassified until 1959 [389]. British Prime Minister Winston S. Churchill ordered all British documents to be purged, listing mustard gas deaths as “burns due to enemy action”. It was not until 1986 that the British Government finally acknowledged that British servicemen had been exposed to mustard gas during the Bari raid and amended survivors’ pensions accordingly.

While therapeutic exploitation of the cytotoxic effect of mustard gas on bone marrow and lymphoid tissues was suggested in 1935 [390], it would take another 15 years before awareness of the 1943 Bari incident and of the Yale group’s 1946 publication would prompt an intense search for anti-cancer agents. Of the thousands of compounds generated and tested in animal models, nitrogen mustard, or NH₂, emerged as the first agent with anti-cancer activity similar to its parent compound but with less toxicity. Nitrogen mustard is still available today under the brand name Mustargen^®. With the lifting of the US OSRD publication ban in 1946, a series of clinical trial reports demonstrating the therapeutic activity of nitrogen mustard in a variety of human malignancies [391–394], accelerated development of numerous derivatives with anti-cancer activity. Many are still in use today, including chlorambucil or Leukeran^®, cyclophosphamide or Cytoxan^®, Iphosphamide or Ifex^®, and melphalan or Alkeran^®. However, initial enthusiasm was tempered by the transient nature of tumor responses and the inescapable relapses. It would take 25 years of trial and error to discover the optimal utilization of nitrogen mustard derivatives that, combined with other drugs (vincristine, procarbazine, and prednisone), would prove capable of inducing prolonged, disease-free survival in most patients with Hodgkin’s disease [395]. However, the drug screening approach can be traced back to Murray Shear of the Office of Cancer Investigations at the PHS, who, in 1935, organized a model drug screening program [396] that ultimately would test over 3,000 compounds in the murine S37 mouse model with the collaboration of US and international colleagues. However, only two compounds progressed to clinical trials but proven too toxic, and the program was abandoned in 1953. In the meantime, the success of penicillin in the treatment of WWII-related wound infections led to a large-scale screening program of potential antibiotics agents, and recognition of the role of p-aminobenzoic acid in the anti-streptococcal activity of sulfa drugs [397] led to a rational proposal for a new direction in drug development [398]. In the same timeframe, folic acid deficiency was found to produce a nitrogen mustard-like effect on the bone marrow, which led to the synthesis of anti-folic acid antagonists Aminopterin [399] that, in 1948, induced the first remissions in acute childhood leukemia [400] and the first cure of widespread gestational choriocarcinoma a few years later [401], and Amethopterin (today’s Methotrexate) [402]. These successes stimulated the synthesis of numerous purines and pyrimidines antagonists, including 6-mercaptopurine [403] and 6-thioguanine [404], both still in use today. While such successes vindicated cogent and organized research, serendipity played a pivotal role in the discovery of numerous cancer drugs, including Vinca alkaloids, epipodophyllotoxins, and platinum, as well as X-rays, penicillin, and many other discoveries [405]. In fact, cancer drug development via mass screening of thousands of natural and synthetic compounds is a process pioneered by Paul Ehrlich at the turn of the twentieth century in his 7-year quest to discover anti-microbial agents. Such a step-by-step approach had its critics in prominent researchers as the National Cancer Act of 1971 was being debated. Sidney Farber, its main detractor, stated at a congressional House Health Subcommittee hearing,

It is not necessary for us to make great progress in the cure of cancer, for us to have the full solution of all the problems of basic research. The history of Medicine is replete with examples of cures obtained years, decades and even centuries before the mechanism of action was understood for these cures [406].

Four decades later, the process of anti-cancer drug development remains mostly anchored on this century-old, conceptually antiquated, technically inefficient, labor intensive, costly, and low-yield “hit-and-miss” screening approach engineered and sponsored by the NCI. Indeed, in a massive, highly complex, and far-reaching undertaking, the NCI’s Developmental Therapeutics Program (DTP) has operated a repository of natural and synthetic products that have been evaluated as potential anticancer agents for over 30 years. The repository, run by a private contractor, has accumulated over 600,000 compounds gathered from the world over. Additionally, since 1986 over 50,000 samples of plants and over 10,000 samples of marine invertebrates and algae from tropical and subtropical waters were added to the repository. The potential anti-cancer activity of each sample is assessed according to its capacity to inhibit the growth of 60 cancer cell-lines (NCI-60) representing leukemia, melanoma, and cancers of the lung, colon, brain, ovary, breast, prostate, and kidney as part of NCI’s “In vitro Cell Line Screening Project” [407]. This project, fully implemented in 1990, has screened approximately 2,500 compounds per year using sequential steps, as follows: New compounds are first pre-screened for in vitro activity against 3 human cancer cell lines. If the growth of at least one cell line is inhibited, the compound is tested against each of the cell lines included in NCI-60. If one or more cell lines are killed, or their growth inhibited at very low concentrations, or if the compound has a unique mechanism of action, it progresses to the next step. At this point, the compound is tested against a standard panel of 12 tumor cell lines placed in individual “hollow-fibers” (small tubes that retain cells but are permeable to the compounds tested) and implanted in mice. Implanted mice are then administered the compound at two different doses, and 4 days later, the hollow-fibers are retrieved and analyzed for cell density. Agents that retard cell growth in implanted hollow-fibers are tested in mice transplanted with specific human cancers. Compounds that kill or inhibit tumor growth after approximately 30 days with minimal animal toxicity become eligible for pharmacology and toxicology studies in animal models and in humans, and, if successful, become eligible for clinical trials (described in the section Clinical trials in Chap. 10).

NCI’s DTP was expected to expose growth inhibition patterns that would unveil groups of agents with distinct mechanisms of actions that in turn might reveal their molecular targets. However, no existing laboratory method can accurately predict the anti-cancer efficacy of a particular chemical and, despite high hopes and years of labor-intensive and costly search, relatively few clinically useful new cancer drugs emerged from NCI’s DTP. Indeed, of 70,702 compounds screened between 1990 and 1998, 6,452 showed potential in vitro activity, 1,546 were chosen for testing in mice, 79 revealed some activity against human tumor cells, of which 10 (or 1.4 per 10,000 screened agents) were eligible for toxicity trials in animals and humans. Yet, according to NCI, “DTP has played an intimate role in the discovery or development of more than 40 U.S.-licensed chemotherapeutic agents, with the rest coming directly from the pharmaceutical industry” [408]. At this writing (May 2013), drugs still in use that can be partly traced to this trial-and-error drug discovery process range from Chlorambucil (1957), to Vincristine (1963), to Hydroxyurea (1967), to Cytosine arabinoside (1969), to BCNU (1977), to Etoposide (1983), to Mitoxantrone (1987), to Carboplatin (1989), to Fludarabine (1991), to Taxol (1992), to Erbitux (2004), and Erbulin (2010) [409]. Cytosine arabinoside, inspired by C-nucleoside derived compounds isolated from the Caribbean sponge Cryototheca crypta, and its fluorinated derivative Gemcitabine are the only cancer drugs rising from the sea. This extremely expensive, labor-intensive, and low-yield drug development approach gives additional meaning to the view, expressed at the turn of the century, that,

The fields and forests, the apothecary shop and temple have been ransacked for some successful means of relief from this intractable malady. Hardly any animal has escaped making its contribution in hide or hair, tooth or toenail, thymus or thyroid, liver or spleen, in the vain search for means of relief [410].

More importantly, all drugs generated by this discovery process are cancer non-specific, cytotoxic agents toxic both to cancer and normal cells. Additionally, they exhibit a narrow “therapeutic window”⁵ that renders them largely inefficacious against cancer. Attempts to enhance anti-cancer activity while minimizing toxicity have achieved neither, as described below.

When I published the War on Cancer in 2005, there were 76 FDA-approved anti-cancer (or anti-neoplastic) drugs. Seventeen of these had been classified by the WHO as “essential” for the treatment of “curable cancers and those cancers where the cost-benefit ratio clearly favors drug treatment” [411]. All 17, developed between 1953 and 1996, are now generic drugs available at low cost. Missing from the list were several newer, more expensive proprietary drugs, notably Imatinib Mesylate and Trastuxumab. Drugs included in a second and third groups, including most of the newer, more expensive drugs, were described as having “some advantages in certain clinical situations” and “not essential for the effective delivery of cancer care”, respectively [412]. Likewise, the WHO has included 5 new drugs in its 2011-updated list of “essential antineoplastic” drugs [413], but, once again, all are cytotoxic agents discovered between 1953 (Methotrexate) and 1996 (Docetaxel). Whether old or new, most cancer drugs in clinical use have anti-proliferative rather than anti-cancer activity, affecting proliferative cancer cells but also normal tissues with high rates of cell turnover. As a result, their therapeutic window is modest and side effects are the norm. With very few exceptions, most of these drugs were discovered by trial and error (i.e., synthetic analogs of the anthracycline antibiotic Daunorubicin), inference (i.e., Nitrogen mustard, a by-product of mustard gas), or serendipity (i.e., Mitoxantrone, a derivative of Ametantrone, a coal-tar derivative originally intended as an ink). For decades, agents initially developed to treat infections but discarded because of excessive toxicity, especially to highly proliferative bone marrow and intestinal lining cells, became prime candidates for screening for anti-cancer activity. Early examples of this strategy include Dactinomycin [414], the second antibiotic discovered after Penicillin, and other so-called anti-tumor antibiotics still in use today, such as Mitomycin-C, Daunorubicin, Mithramycin, Doxorubicin, Bleomycin, Mitoxantrone, and Idarubicin.

By 1951, thiopurines 6-thioquanine and 6-mercaptopurine had shown activity not only against acute leukemia but also in a variety of disorders, including herpes, gout, and as immunosuppressive agents in organ transplantation [415]. Based on observation of greater uracil uptake by rat hepatoma cells than by normal tissue, researchers at the University of Wisconsin synthesized 5-fluorouracil (5-FU) by fluorination of a uracil pyrimidine base [416]. This agent proved active against a range of solid tumors and remains the cornerstone for treating colon cancer today. Shortly thereafter, choriocarcinoma became the first curable invasive cancer and the future of chemotherapy seemed assured [417]. However, failure to replicate these successes in other cancers led researchers to search for exploitable differences between normal and cancer cell biology for therapeutic gain, focusing attention on the cell cycle. This, despite admonitions by a pioneer cancer researcher who warned,

Those who have not been trained in chemistry or medicine may not realize how difficult the problem of cancer treatment really is. It is almost, not quite, but almost as hard as finding some agent that will dissolve away the left ear, say, yet leave the right ear unharmed: so slight is the difference between the cancer cell and its normal ancestors [418].

Nevertheless, it was discovered that while all cancer drugs seemed to block cell replication, they did so via inhibiting specific phases of the cell cycle (phase-specific drugs), or acting directly or indirectly on DNA, RNA or the cell membrane (not phase-specific drugs). Phase-specific drugs exert their effect either during the S or DNA synthesis phase, the M or mitotic phase, or during the G1 or G2 phases of the cell cycle. Hence, anti-tumor drugs are sub-classified into several distinct categories according to their mechanism of action. Alkylating agents, such as the nitrogen mustards, nitrosoureas, and the platinum subgroups, are not phase-specific drugs but impair cell replication by forming bonds with DNA, RNA and certain proteins. Anti-tumor antibiotics, such as Dactinomycin, Doxorubicin, and Bleomycin are non-phase-specific agents with a complex mechanism of action. These agents, best exemplified by the Anthracyclin subgroup, can intercalate between base-pairs of DNA, disrupting DNA replication and RNA transcription, produce single- and double-stranded DNA splits, damage DNA through creation of free radicals, and possibly disrupt cell membranes; Antimetabolites, such as Methotrexate, Cytarabine, and 5-fluorouracil, are S-phase specific agents that are structural analogs to normally occurring metabolites involved in DNA synthesis. They exert their cytotoxic activity by competing with metabolites involved in key RNA or DNA regulatory enzymes or by directly substituting metabolites normally incorporated in the RNA or DNA molecules themselves; Mitotic inhibitors, best represented by the Vinca alkaloids (Vincristine and Vinblastine), bind tubulin, a cell protein that polymerizes to form the microtubular filaments along which chromosomes migrate during mitosis (cell division). Vinca alkaloids prevent tubulin polymerization, resulting in arrest of cell division in metaphase, followed by lysis; finally, a number of older drugs, such as L-asparaginase, and many of the newer ones, such as the monoclonal antibody and the immunotoxins groups, not to mention cancer-active hormones, have mechanisms of action that do not fit into any of these categories. Furthermore, as in any biologic process, the factors and steps involved in cytotoxic cell death are multifaceted and the result of a multitude of contributing intra- and extra-cellular signals, and other factors peculiar to a particular cancer and a given host. For example, one would expect that an antimetabolite purine analogue that blocks DNA synthesis via inhibiting DNA polymerase alpha, ribonucleotide reductase, and DNA primase, Fludarabine would be very active against cancers with high growth rates. Instead, it is active against and FDA-approved for the treatment of adult patients with B-CLL, a human malignancy characterized by one of the lowest growth rates where the main defect is impaired apoptosis that causes the accumulation of long-lived malignant cells. Additionally, cell cycle kinetics alone fail adequately to describe tumor growth or to explain unexpected tumor response patterns to anti-tumor drugs. Indeed, the cell cycle time for normal cells is 1–2 days versus 2–3 days in most cancers [419], and the proliferative cell pool in CML is up to tenfold greater than in AML, despite its much less aggressive course amenable to much longer survival. The explanation for this apparent incongruity rests on the fact that CML myeloblasts differentiate into mature, functional, and short-lived granulocytes, whereas AML myeloblasts do not, and, given their high proliferative rate and longer life span, accumulate rapidly [420, 421].

Thus, as clinical trial results often failed to confirm anti-tumor drug efficacy predicted by their presumed mechanisms of action and by cell kinetics data, new hypotheses were postulated to explain the observed discrepancies. H.E. Skipper and colleagues of the Southern Research Institute proposed an early and influential hypothesis based on the L1210 mouse leukemia model, a versatile animal tumor screening system adopted by NCI. The hypothesis included two laws widely regarded as ground-breaking [422]. The first law established that the doubling time of proliferating cancer cells is constant and exponential. The second law postulated that anti-tumor drugs follow “first-order kinetics”; that is, the fraction of cells killed by a given drug at a given dose in a given tumor is constant regardless of the size or state of the cancer. According to this view, a drug that kills 90 % of cells of a tumor will do so each time it is administered, whether the tumor is very large or microscopic. However, clinical observations were at variance with Skipper’s laws, leading to the Mendelsohn’s concept of growth fraction [423] and the Goldie-Coldman hypothesis on drug resistance, which is based on a mathematical model that predicts that tumor cells mutate to a resistant phenotype at a rate dependent on their intrinsic genetic instability [424]. Indeed, most human tumors do not expand exponentially and respond to chemotherapy, following patterns far more complex than suggested by a simplistic first order kinetics model.