INTRODUCTION:

In the treatment of cancer, one of the basic assumptions is that all malignant cells should be destroyed, removed or neutralized to achieve cure. With experimental animal tumors, it has been proved that a single tumor cell can grow and eventually kill the host².

Four major therapy modalities exist today which can be used in attempt to bring about the requisite malignant cellular reduction as

(A) Surgery

(B) Radiotherapy

(D) Immunotherapy and other biology therapy.

Cancer can be classified into (I) solid tumors and (II) hematological malignancies. In solid tumors, surgery and radiotherapy are the traditional primarily chosen treatments though several cancer chemotherapeutic drugs are also being used now a days. In hematological malignancies, chemotherapy is the treatment of choice at diagnosis.

Strategy of cancer treatment:

(A) Surgery:

Surgery is considered as the oldest treatment for cancer and until few decades of this century was the only treatment modality that could cure patients particularly when the disease was localized. The modern era of elective surgery for visceral tumors began in frontier America in 1809 when E. MacDowell removed a large ovarian tumor from a patient. However, after the introduction of general ether anesthesia in 1846 by Dr. J. Warren and principle of antiseptics by J. Lister in 1869 respectively surgical oncology was greatly increased. W.S. Halsted elucidated the principles of en block resections for cancer as exemplified by his development of the radical mastectomy in 1890 followed by radical prostatectomy in 1904 by H.H. Young and radical hysterectomy in 1906 by Dr. E. Wertheim. Developments in the technical surgery continue at a rapid pace and modern technical innovations are continuing to extend significantly the surgeon’s reach into a variety of areas including the most difficult and distant parts of the body³.

Surgical removal has obvious advantages and disadvantages. Once a block of tissues has been removed it can do no harm in a specimen container but equally well there can be no direct effect of surgery due to its local and regional applications. Recurrence of the disease often results due to distant metastasis. Hence other treatment modalities particularly radiotherapy and chemotherapy are being practiced simultaneously as pre- and post-operative therapy.

(B) Radiotherapy :

This is considered as the second main and oldest method of treatment. After the discovery of X-rays by William Conrad Roentgen on 8 November, 1895 the first application of X-rays for therapeutic purpose was made in a female patient in the treatment of her breast cancer on January 29, 1896. Antoine Henri Becquarrel on 1st march 1896 subsequently discovered gamma radiation. The first case of cure of a malignant tumor by radiotherapy alone was reported when a female patient received 150 radiations over 9 months for the treatment of squamous cell carcinoma of her nose in 1899. In mid–1930s, radiotherapy was beginning to be used as an adjuvant to radical mastectomy. With the introduction in the 1950s of ^6oCo and ¹³⁷Cs as sources of therapeutic radiation and the developments of betatrons and linear accelerators, the ability to provide an adequate target dose avoiding at the same time undesirable side-effects was greatly improved. This was the beginning of modern radiotherapy. It is often recognized that some tumors like Wilm’s tumor, Hodgkin’s disease, neuroblastoma etc are sensitive and show good response to the radiotherapy even with exposure with low degree radiations⁴.

There are certain disadvantages as (1) Radiotherapy has no direct action on distant tumors and is used as local therapy. (2) The therapeutic dose may also damage and kill the adjoining normal tissues. (3) It cannot be applied to the sites which are very susceptible and sensitive to radiation damage. (4) Some bulky and solid tumors do not respond to radiotherapy. (5) There are hazards including development of secondary neoplasms in radiation treatment etc.

However, with the development of modern delivery techniques like the availability of newer radiation sources including computer assisted dosimetry, improved tumor localization using computer topography, some of these disadvantages have been controlled to some extent and radiation has become more selective and is becoming popular in the treatment and control of many cancers.

(C) Chemotherapy:

At the turn of this century, cancer was regarded as a disease that began locally and that spread progressively to the regional lymph nodes and only at a late stage via the blood streams to distant parts of the body. After the introduction of radiotherapy in the 1930s this form of local treatment was also added to the therapeutic armamentarium. Many cancers metastasize by the bloodstream at an early stage and patients may die from metastatic disease even though the primary sites remains free of tumor by the combination of treatment modalities as surgery and radiotherapy. Attempts at chemical treatment of cancer were recorded⁵ as long ago as the first century A.D. It is of interest that colchicine, prepared from the bark of the cinchona tree, was first used as an anticancer agent. The implications of the observation⁶ that soldiers dying in the First World War after exposure of yellow mustard gas had aplasia of lymphoid organs and bone-marrow were only appreciated some 20 years later after the confirmation of the Krumbharrs’ observation by the two renowned scientists namely Goodman and Gilman. The first ever-successful drug nitrogen mustard was introduced into clinical practice in the treatment of lymphomas and leukemias from 1946 onwards.

It triggered search for new anticancer agents and a variety of sources have been explored through random screening of thousands of synthetic chemicals, microbial products, plant and animal extracts and various other sources. Within 1960, a series of anticancer compounds were developed and tested on transplantable tumors in rodents. These are still playing important roles in the control of a number of cancers. It is interesting that many effective drugs were discovered in research programmes that had little or no relation to cancer⁷. While the alkylating agents originated from chemical warfare, antimetabolite drugs came from nutritional and antimicrobial research or the investigation of nucleic acid metabolism.

(i) Alkylating agents:

The alkylating agents are anti-tumor drugs that act through the covalent bonding of alkyl groups (one or more saturated carbon atoms) to cellular molecules. Historically, the alkylating agents have played an important role in the development of cancer chemotherapy. The nitrogen mustards, mechlorethamine [HN₂, nitrogen mustards] and tris chloroethyl) amine [HN₃], were the first non hormonal agents to show significant antitumar activity in humans. Some of the commonly used drugs of various subclasses are described as per Table-1³.

Table-1: Classification of alkylating agents

Subclass	Drugs
Nitrogen mustards	Mechlorethamine (HN₂), Chlorambucil, Melphalan Cyclophosphamide, Ifosfamide
Nitrosoureas	Carmustine (BCNU), Lomustine(CCNU), Semustine (Me-CCNU), Streptozotocin, Chlorozotocin
Ethyleneimines	Triethylene melamine (TEM), Triethylene
Alkyl sulfonates	Busulfan
Epoxides	Dibromomannitol, Dibromodulcitol
Dialkyltriazenes	Dacarbazine (DTIC)

Chemistry and Mechanism of alkylating reactions:

Traditionally, alkylating reactions have been classified as S_N1 (nucleophilic substitution first-order) or S_N2 (nucleophilic substitution, second-order) (Drawing 1). In the S_N1 reaction there is an initial formation of a highly reactive intermediate, followed by the rapid reaction of this intermediate with a nucleophil to produce the alkylated product. In this reaction the rate limiting step is the initial formation of the reactive intermediate. Thus, the reaction exhibits first-order kinetics with regard to the concentration of original alkylating agent and the rate is essentially independent of the concentration of the substrate, hence the designation S_N1.

Fig. – 1: S_N1 and S_N2 reactions

The S_N2 alkylation reaction represents a bimolecular nucleophilic displacement. The rate of this reaction is dependent on the concentration of both the alkylating agent and the target nucleophile. Therefore, the reaction follows second-order kinetics. The term S_N1 and S_N2 are defined kinetically but normally are used in reference to the mechanism of action.

(ii) Antimetabolites:

Antimetabolites are agents that interfere with normal metabolism due to their structural similarity with normal intermediates in the synthesis of RNA and DNA precursors. They either serve as substrates for enzymes, inhibit enzymes, or do both. Due to differences in metabolism between normal cells and cancer cells, several antimetabolites have the potency to act with a certain degree of specificity on cancer cells.

Table-2: Classification of antimetabolites

Subclass	Drugs
Folate	Methotrexate (MTX)
Purine analogues	6-Mercaptopurine(6-MP), 6-Thioguanine (6-TG), Azathiopurine
Pyrimidine analogues	5-FU, Cytosine arabinoside (Ara C)

MTX blocks folic reductase, 5-FU blocks thymidylate synthetase both in turn inhibit methylation of deoxyuridylic to thymidylic acid. 6-MP, 6-TG and also MTX block purine ring biosynthesis. Ara C inhibits reduction of cytidilic acid to deoxycytidylic acid and also inhibits DNA polymerase. All these compounds find application in the treatment of several human cancers⁴.

(iii) Plant products:

The plant kingdom has long served as a prolific source of useful drugs in the treatment of various diseases. The search of anti-tumor agents from the plant kingdom has been in active pursuit after 1950s but the greatest impetus came after the discovery of vincristine and vinblastine from Catharanthus roseus around 1960. The folklore knowledge of its hypoglycaemic activity stimulated the investigation of this plant which ultimately led to the discovery of above active principles. Later on another semi-synthetic derivative namely vindesine has been developed. Cancer research has led to a reassessment of many primitive plant cure and a massive search for the anti-tumor principles have provided many active products belonging to the following classes of chemical compounds as diterpenes, lignans, quassinoids, ansamacrolides and alkaloids some of which are listed below:

Table-3: Classification of plant products

Subclass	Drugs
Vinca alkaloids	Vincristine, Vinblastine, Vindesine
Epidophyllotoxins	Etoposide (VP-16), Teniposide (VM-26)
Taxol derivatives	Taxol, Taxotere
Dysoxylum bineclariferum Hook.f.(Meliaceae)	Flavopiridol
Combretum caffrum (Eckl.and Zeyh.)	Combretastatins
Raphanus sativas L.(Brassicaceae)	Roscovitine
Aglaila sylvestre	Sylvestrol
Erythroxylum pervillei	Pervilleine A
Centaurea schischkinii	Schischkinnin

In the massive screening program, NCI, USA, has played the pioneer role and they have screened thousands of plant products. Early drugs colchicine, podophyllotoxin, their derivatives and semi-synthetic products except VM-26 and VP-16 has very limited clinical application.

Quite recently the introduction of taxol group of compounds as drugs has led to great interest and boost among scientists and physicians. Taxol obtained from the pacific yew tree Taxus brevifolia has demonstrated significant activity in patients with refractory cancer including some solid tumors as ovarian cancer progressing on prior therapy with cisplatin. Activity has also been shown in advanced breast cancer⁵.

(iv) Anti-tumor antibiotics:

The anti-tumor antibiotics are natural products usually derived from fermentation broths mostly from streptomyces species as per the table-4.

Table: 4 Classification of anti-Tumor antibiotics:

Subclass

Drugs

Anthracyclins

Doxorubicin(adriamycin), Daunorubicin

Others

Actinomycin D, Bleomycin, Mitomycin,

Mithramycin

Most of the available anti-tumor antibiotics are DNA binders and intercalators. One of the most unusual structures that has antitumor activity is bleomycin, a mixture of water-soluble small molecular-weight peptides isolated from the fungus Streptomyces verticullus. The primary action of bleomycin is to produce single- and double-strand breaks in DNA. The first anthracyclins put in clinical use are daunomycin and doxorubicin again produced from the Streptomyces species. As anti-tumor agents, they are matched only by alkylating agents in terms of their clinical usefulness and both of them find application in the treatment of a number of malignancies⁶.

(v) Hormones and related agents:

Beatson introduced excision of endocrine glands to control human cancer in 1896 but it was not until 1941 when Huggins et al first reported the dramatic effect of orchiectoimy in man with cancer of the prostate. In the earlier year, Huggins demonstrated that in the dog, the shrinkage of the gland and cessation of the secretion followed castration and that these effects could be reversed by the administration of an androgen. Effective hormone therapy begun in 1941 with the treatment of cancer of the prostate with diethylstilbestrol (DES) and with estradiol dipropionate by Herbst.

The early success with hormone therapy led to the synthesis of a number of new steroids and other related structures with hormone activity that show less side effects. At the same time it was also felt that it is futile to hope that hormone therapy alone will ever cure any form of cancer because relapse of responders inevitably occurs. This may be due to the reason that hormone-dependent tumors are made up of a heterogenous cell population that includes cells that are not hormone-dependent. The most useful adrenocorticosteroid compounds that are used now a days are prednisone and prednisolone used in combination with other drugs in the treatment of various lymphomas particularly Hodgkin’s disease, leukemias etc. Among the antiestrogens and androgens class the commonly used drug is tamoxifen, which may induce remission upto 30% of postmenopausal women with metastatic breast-tumors⁷.

(vi) Miscellaneous agents:

This group includes those chemical compounds whose mechanism of action is obscure or whose mechanism of action is different from the above classes. Some of the examples are described in the table- 5.

Table-5: Classification of miscellaneous agents

Subclass	Drugs
Enzymes	L-Asparaginase
Metal compounds	Cisplatin, Carboplatin
Others	Procarbazine, Hydroxyurea, Hexamethylmelamine (HMM), Pentamethylmelamine (PMM), Amsacrine etc.

(D) Immunotherapy and Biological therapy:

It has been observed that immunologic mechanisms influence the development and growth of malignant tumors. In contrast to chemotherapy which follows ‘first order’ kinetics, immunotherapy follows ‘zero order’ kinetics which indicates that specific number of antibodies would be required for each tumor cell lysis and thus it is possible theoretically that all the tumor cells including the last one can be killed. But it has been found that it can tackle relatively small number of cells and hence the property of last cell kill can be successfully exploited only after reducing tumor bulk by other treatment modalities.

Classical immunotherapy of tumor has been attempted for nearly 100 years without success. Starting from the earlier experiments of Dr. E. Klein in 1969 on the treatment of human skin cancer by hypersensitivity reaction yielding encouraging results, this area is rapidly growing. The concept of immunochemotherapy by drug conjugated with antibody seem to bring rational approach as this will result in selective tumor cell kill without affecting the normal cells of the host is gaining popularity. Until 1980, the absence of purified products of the interferons, interleukins, tumor necrosis factor (TNF) and other lymphokines, precluded serious clinical study⁸.

Gene therapy is a new therapeutic approach in which a functioning gene is inserted into a cell to correct a metabolic abnormality or to introduce a new function. It is considered to play a significant role in the treatment of human cancers in the future years to come. In recent years, a variety gene therapy approaches are being investigated in a number of laboratories.

Newer anticancer drugs:

HDAC inhibitor:

Also in recent years, there has been an effort to develop HDIs as a cancer treatment or adjunct^9,
10. The exact mechanisms by which the compounds may work are unclear, but epigenetic pathways are proposed¹¹. Richon et al. found that HDAC inhibitors can induce p21 (WAF1) expression, a regulator of p53's tumor suppressor activity. HDACs are involved in the pathway by which the retinoblastoma protein (pRb) suppresses cell proliferation¹². The pRb protein is part of a complex which attracts HDACs to the chromatin so that it will deacetylate histones¹³. HDAC1 negatively regulates the cardiovascular transcription factor Kruppel-like factor 5 through direct interaction¹⁴. Estrogen is well-established as a mitogenic factor implicated in the tumorigenesis and progression of breast cancer via its binding to the estrogen receptor alpha (ERα). Recent data indicate that chromatin inactivation mediated by HDAC and DNA methylation is a critical component of ERα silencing in human breast cancer cells¹⁵.

Examples:

• Vorinostat was licenced by the U.S. FDA in October 2006 for the treatment of cutaneous T cell lymphoma (CTCL).

• Romidepsin (trade name Istodax) was licenced by the US FDA in Nov 2009 for cutaneous T-cell lymphoma (CTCL),

• Panobinostat is under investigation for various cancers including cutaneous T cell lymphoma (CTCL).

• Valproic acid is under investigation for various cancers including leukemia.

• Mocetinostat (MGCD0103) is undergoing clinical trials for treatment of various cancers (including follicular lymphoma, Hodgkin lymphoma and acute myeloid leukemia).

Proteasome inhibitors:

Proteasome inhibitors are drugs that block the action of proteasomes, cellular complexes that break down proteins, like the p53 protein. Proteasome inhibitors are being studied in the treatment of cancer.

Examples

• In 2003, bortezomib was the first proteasome inhibitor to be approved for use in the U.S.

• Disulfiram has been proposed as another proteasome inhibitor.^{16, 17, 18}

• Epigallocatechin-3-gallate has also been proposed.¹⁹

• Salinosporamide A has started clinical trials for multiple myeloma.

Telomerase inhibitor:

Telomerase, the ribonucleoprotein enzyme maintaining the telomeres of eukaryotic chromosomes, is active in most human cancers and in germline cells but, with few exceptions, not in normal human somatic tissues. Telomere maintenance is essential to the replicative potential of malignant cells and the inhibition of telomerase can lead to telomere shortening and cessation of unrestrained proliferation. We describe novel chemical compounds which selectively inhibit telomerase in vitro and in vivo. Treatment of cancer cells with these inhibitors leads to progressive telomere shortening, with no acute cytotoxicity, but a proliferation arrest after a characteristic lag period with hallmarks of senescence, including morphological, mitotic and chromosomal aberrations and altered patterns of gene expression. Telomerase inhibition and telomere shortening also result in a marked reduction of the tumorigenic potential of drug-treated tumour cells in a mouse xenograft model.

Examples:

· Two examples from this class of compounds, designated BIBR1532 {2-[(E)-3-naphtalen-2-yl-but-2- enoylamino]-benzoic acid} and BIBR1591 {5-morpholin- 4-yl-2-[(E)-3-naphtalen-2-yl-but-2-enoylamino]-benzoic acid}.

Antiangiogenic drugs:

A substance in the body called Vascular Endothelial Growth Factor (VEGF) is responsible for the growth of new blood vessels. It promotes this growth by stimulating the endothelial cells, which form the walls of the vessels and transport nutrients and oxygen to the tissues. Evidence shows that when the retinal pigment epithelial (RPE) cells begin to wither from lack of nutrition (a condition called "ischemia"), the VEGF goes into action to create new vessels. This process is called "neovascularization," and it acts as a restorative function in other parts of the body. In the retina, however, the vessels do not form properly, and leaking results. This leakage causes scarring in the macula and eventual loss of central vision. Antiangiogenic drugs prevent the VEGF from binding with the receptors on the surface of the endothelial cells. In most cases, the drugs are injected into the vitreous of the eyeball, then pass into the subretinal space, where the vessels proliferate. Neovascularization is then blocked, preventing bleeding into the retina.

Examples.

Macugen (pegaptanib sodium), Lucentis, Tryptophanyl-tRNA synthetase (TrpRS), Combretastatin A4 Prodrug (CA4P), Avastin (bevacizumab), Sirolimus (rapamycin), Endostatin etc.

COX-2 inhibitors as an Antiangiogenic drug:

The formation of new blood vessels by angiogenesis to provide adequate blood supply is a key requirement for the growth of many tumors. While normal blood vessels expressed the COX-1 enzyme, new angiogenic endothelial cells expressed the inducible COX-2. We evaluated the role of COX inhibitors in the mouse corneal micropocket assay in which angiogenesis is driven by the addition of a Hydron pellet containing basic fibroblast growth factor (bFGF). Neovascular areas were measured with a slit lamp five days after pellet implantation into the corneal stroma. All animals containing implants with bFGF (90 ng) developed intensive areas of neovascularization, whereas the controls implanted with the Hydron pellet alone did not. Indomethacin (a nonselective COX-1/COX-2 inhibitor) and SC-236 (a COX-2-selective inhibitor) inhibited angiogenesis in a dose-dependent manner. Importantly, the indomethacin-treated mice developed severe gastrointestinal toxicity at the efficacious dose of 3 mg/kg/day. By contrast, gastrointestinal lesions were not observed, and platelet COX- 1 activity was unaffected, at anti-angiogenic doses of SC-236 (1–6 mg/kg/day). Furthermore, a COX-1-selective inhibitor, SC-560, was ineffective at doses up to 10 mg/kg, a dose that completely blocked platelet COX-1 activity in these mice. SC-236 was also effective in reducing angiogenesis driven by bFGF, vascular endothelium growth factor (VEGF), or carrageenan in the matrigel rat model. Finally, in several tumor models, SC-236 consistently and effectively inhibited tumor growth and angiogenesis. This novel antiangiogenic activity of COX-2 inhibitors indicates their potential therapeutic utility in several types of cancer.

CONCLUSION:

The factors which determine specific types of cancer are age, sex, race, genetic predisposition and the carcinogens exposed to the environment. A number of chemicals have been clearly shown to be carcinogenic e.g. Tobacco smocks, benzene & some of its derivatives and some dyes as seen in animal tests. Whether may be the cause or the site, it is a disease in which a shift has taken place in the process of cell proliferation and differentiation. In most cases the cells can proliferate excessively and form local tumours that can compress or invade adjacent structure. Such neoplasms with the characteristic of only local growth are termed benign.Neoplasm with the additional characteristics of invasiveness and /or the capacity of metastasis is classified as malignant. Above such tumour stem cells can thus express colonogenic or colony-forming capability. The invasive and metabolic processes as well as a series of metabolic abnormalities resulting from cancer cause illness and eventual death of the patient unless the neoplasm can be eradicated by the above treatment.

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Received on 27.02.2011 Accepted on 20.05.2011

Asian J. Pharm. Tech. 1(2): April-June 2011; Page 28-33

(A) Surgery:

(C) Chemotherapy:

(iv) Anti-tumor antibiotics:

Procarbazine, Hydroxyurea, Hexamethylmelamine (HMM), Pentamethylmelamine (PMM),

Amsacrine etc.