OHO-08(93-134) 11/16/02 11:19 AM Page 129 THE ROLE OF UNSEALED RADIONUCLIDES 129 Applications in diagnosis and follow-up Bone scintigraphy Bone scintigraphy is normally performed 2 \200\2234 hours after the injec tion of 550 MBq of 99mTc-labeled methylene disphosphonate (99mTc- medronate, MDP) or hydrox ymethylene disphosphonate (99mTc- oxidronate, HDP). Multiple planar images or a whole-body survey of the skeleton are obtained. Skeletal scintigraphy has high sensitivity for the det ection of primary and metastatic bone lesions. In the absence of reactive osteoblastic acti vity, the lesion itself may appear on the bone scan as a \200\230cold \200\231 defect. High sensitivity (80 \200\223100%) has been reported in patients with breast carcinoma, pro static carcinoma, bronchogenic carcinoma, gastric carcinoma, osteogenic sarcoma, cervix car cinoma, Ewing \200\231s sar- coma, head and neck carcinomas, neuroblastoma, and ovarian car ci- noma. Lower sensitivity, around 75%, has been found in melanoma, small-cell lung tumors , Hodgkin \200\231s disease, renal-cell carcinoma, rhabdomyosarcoma, multiple myeloma, and bladder carcinoma. Liver scintigraphy The conventional liver scintigram is performed after IV administra- tion of 50 \200\223100 MBq of 99mTc-labeled colloidal particles varying in size from 0.3 \200 \2231.0 mm. The radioactive agent accumulates in the cells of the reticulo-endothelial syst em by phagocytosis. The reticulo- endothelial system cells are homogeneously distributed in the liver and spleen, and to a lesser extent in bone, marrow, and lungs. The most common indication for liver scintigraphy used to be the detection of space-occupyi ng lesions. Nowadays, ultrasonography and CT are generally considered to be more effective modalities than scintigraphy. The solitary non-cystic lesion may pose a clinical problem. Non- invasive differentiation b etween haemangioma and metastasis or between hepatoma, hepatocellular carcinoma, follicular nodular hyperplasia, and metastasis is important for the clinical management of the patien t. Scintigraphic studies with 99mTc-labeled erythrocytes, iminodiacetic acids (IDA), or wit h 67Ga can be used as non-invasive methods for a more speci \201c characterization of the tumor in addi- tion to colloid scintigraphy. Thyroid scintigraphy 131I as radioiodine, 123I as sodium iodide, and 99mTc as sodium pertec hnetate are the radionuclides used for scintigraphic visualiza- tion of the thyroid gland. Although 131I is cheap and readily available, its major disadvantages are its long physical half-life and (cid:2)-emissions, resulting in a considerable radiation dose to the thyroid and the gastrointestinal tract. 123I has excellent physical properties for imaging OHO-08(93-134) 11/16/02 11:19 AM Page 130 PRINCIPLES OF RADIATION ONCOLOGY 130 and a physical half-life of 13 hours, but its use is limited due to its cost. 99mTc is trap ped in the thyroid, but is not organi \201ed and washes out from the gland over time. The indications for thyroid scintigraphy in oncology are: (cid:2) Evaluation of a solitary or dominant nodule. (cid:2) Follow-up after surgery for di fferentiated thyroid cancer. Thyroid scintigraphy with 99mTc and 123I can be used to assign prob- ability of malignant disease on the basis of the functional status of the nodule. Hy perfunctioning nodules are almost always benign, but malignancy can be found in 4% of the h ot nodules. The ma- jority of non-functioning nodules represent benign cysts and adeno- mas ; l6 \200\22320% of solitary non-functioning nodules contain thyroid cancer. Scintigraphy of medullary thyroid cancer is possible with 201T1, 99mTc-DMSA-V, 131I- or 123 I-labeled m-iodobenzylguanidine (MIBG), and 111In- or 123I-labeled somatostatin analogs. Th e sensitivity of these methods in detecting the primary tumor or its metastases is generall y low. For the new tracer, 111In-octreotide, the sensitivity for the detection of medullary thyroid cancer is 66%, compared with 35% for 131I-MIBG. The value of 131I-MIBG scintigraph y is to determine whether a patient may bene \201t from therapy with this radiopharmaceuti cal. Imaging of neuroendocrine tumors Phaeochromocytoma and other neural crest tumors can succes sfully be detected with 123I or 131I-MIBG. The molecular structure of MIBG has some similar ity to noradrenaline. The scintigraphic study is done at intervals of 24, 48, and occasiona lly 72 hours after intravenous injections of 370 MBq of 123I-MIBG or 37 MBq of 131I-MIBG. W ith a sensitivity of 92% and speci \201city of nearly 100% in neuroblastoma, MIBG scintigr aphy is a useful technique for the detection, staging, and follow-up of this disease. Addit ionally, it serves as an indicator for potential therapy using 131I-MIBG. Somatostatin is a polypeptide hormone with a short biological half- life. Receptors for thi s hormone are present on many cells of neuroen- docrine origin. A long-acting analog, octre otide, was synthesized for therapy. 111In-octreotide was developed for imaging, performed 2 4 and 48 hours after injection of 111 MBq. The potential clinical value of somatostatin rec eptor scintigraphy is in the detection of occult primary tumors, screening of metastases fo r staging, follow-up of therapy, and selection of patients who might bene \201t from palli ative treatment with unlabelled octreotide. The cumulative sensitivity of 111In-octreotide scintigraphy for the detection of endocrine pancreatic tumors is around 75%, and for carcinoid tumors, around 85%. These tumors are oft en small and not easily recognized on CT or MRI. OHO-08(93-134) 11/16/02 11:19 AM Page 131 THE ROLE OF UNSEALED RADIONUCLIDES 131 Monoclonal antibodies The development of monoclonal antibodies that recognize human tumor-a ssociated antigens has implications for nuclear medicine. Antibodies and fragments (Fab) ca n be labeled with 131I, 123I, 111In, and 99mTc for scintigraphy. In colorectal cancer, scin tigraphy with monoclonal antibodies is potentially valuable in detecting occult disease, lo calizing recurrent disease when serum markers are elevated, and differentiating between via ble tumor and post-surgical \201brosis. The potential roles for radio-immunoscintigraphy in ovarian cancer are in the evaluation of chemotherapy, of pelvic masses after therapy for the presence of viable tumor, and of raised serum markers. Imaging of tumor tissue Nuclear medicine techniques have a limited role in the characteriza - tion of tumors. Modalities such as plain X-ray radiography, ultra- sound, CT, and MRI off er superior anatomical detail. The value of tumor-seeking radiopharmaceuticals lies in the capability of distinguishing viable tumor tissue from a speci \201c residual mass after th erapy and in convenient screening for additional lesions using whole-body imaging. Gallium scintigraphy is usually done 48 \200\22372 hours after the IV administration of 180 MBq 67Ga-citrate. It is of limited value in the initial staging of Hodgkin \200\231s and n on-Hodgkin \200\231s lymphoma, because of the dif \201culty of detecting lesions in the ab domen. In patients with Hodgkin \200\231s disease and other lymphomas, 67Ga scintigraphy, i ncluding SPET is an indicator of residual viable tumor tissue with a better pre- dictive va lue than CT. Thallium-201 is widely used for myocardial perfusion imaging. Over the past few years there has been growing interest in the use of 201T1 in oncology. It may be useful in the differe ntiation between low- and high-grade glial tumors and between tumor recurrence and scar tis sue. Thallium-201 scintigraphy can assess residual tumor via- bility after chemotherapy in bone and soft-tissue sarcomas. Assessment of organ and tissue damage related to cancer therapy Ventilation/perfusion lung scintigraphy Patients will lose a substantial part of their pulm onary capacity after surgery (pneumonectomy or lobectomy). With ventilation/perfusion scint igraphy, quantitative information on regional perfusion and ventilation can be obtained. Ve ntilation/perfusion scintigraphy is also useful in the assessment of pulmonary damage after radiation therapy. Typically, a reduction of perfusion is seen in the irradiated parts. OHO-08(93-134) 11/16/02 11:19 AM Page 132 PRINCIPLES OF RADIATION ONCOLOGY 132 Left ventricular function in cardiac injury Doxorubicin and other anthracycline derivatives are effective chemotherapeutic agents, but one of the major limiting side-effects is cardi otoxicity. An essential aspect in the treatment strategy is serial monitoring of left ventr icular ejection fraction (LVEF) at rest. Radionuclide ventriculography can be performed und er standardized conditions, ensuring a good reproducibility that is essential in serial mea surements. A decline in LVEF is a relatively late symptom of cardiotoxicity. Recently, meth ods to detect directly the myocyte dam- age by 111In-labeled antimyosin scintigraphy have b een developed. Lymphoscintigraphy In patients with breast cancer, the evaluation of acquired lympho- edema caused by radiation therapy or lymph node resection is impor- tant in the assessment of co mpetency of lymphatic drainage and/or to select the location for possible anastomotic surge ry. Therapy with open radioactive sources Targeted radiotherapy using tumor-seeking radiopharma ceuticals has been employed for almost half a century. The radiopharmaceuti- cal should hav e speci \201c af \201nity for tumor tissue with a high target- to-background ratio and lo ng retention time; the radiation emitted by the radioisotope should be suf \201ciently ene rgetic for a therapeutic effect, but be absorbed over a short distance to irradiate the tum or target only. Some of the clinically useful radiopharmaceuticals for therapy are 131I, 89 Sr, 32P, 186Re, 153Sm, and 90Y. Iodine-131 therapy in differentiated thyroid cancer 131I has been used extensively in the t reatment of benign thyroid disease and in differentiated thyroid carcinoma after surgery. 1 31I is used for ablation of the remaining thyroid tissue following total thyroidectomy and for treatment of recurrent and metastatic disease. 131I-m-iodobenzylguanidine therapy in neural crest tumors 131I-MIBG has been used successfu lly for radionuclide therapy of neural crest tumors. Post-therapy scintigrams, one week aft er administration, can be obtained for further documentation. In patients with malignant ph aeochromocytoma, response is achieved in >50% of patients; with neuroblastoma the response rate is 35%. Some success is reported with 131I-MIBG therapy for paraganglioma and medullar y thyroid carcinoma. Bone-seeking radiopharmaceuticals for intractable bone pain Bone metastases occur in up to 85% of patients who have breast, lung, or prostate cancer. Bone-seeking radiopharmaceutical s have pharmaco- OHO-08(93-134) 11/16/02 11:19 AM Page 133 THE ROLE OF UNSEALED RADIONUCLIDES 133 kinetic properties similar to either calcium or phosphate. Strontium-89 (89Sr) is a calcium analog. 32P, 86Re, HEDP, and 153Sm are all phosphate analogs. Application of 32P-orthophosphate for the treatment of bone pain was effective, but bone ma rrow toxicity limited its widespread use. 89Sr was the \201rst radioisotope employed as a systemic treatment for bone metastases in prostate cancer. After IV administration of l50 MBq of 89Sr the radiopharmaceutical is avidly accumulated in areas of high bone turnover, s uch as reactive bone surrounding a metastasis. A tran- sient leucopaenia can be expected af ter 6 weeks. After a single admin- istration of 89Sr, in 75 \200\22380% of patients pain is promptly relieved and progression of further bone disease is delayed, with the response la st- ing 1 \200\2236 months. Rhenium-186HEDP is a new radiopharmaceutical with similar chemistry and biodistribution to 99mTc-labeled diphosphonates. There is a tendency for palliative response to decrease appro ximately 7 weeks following injection in some patients. Haematopoietic toxicity is minimal. Samarium-153-EDTMP preferentially localizes in bone metastases and is rapidly cleared from the blood by the kidneys. Intracavitary therapy Injection of radiopharmaceuticals directly into the pleural cavity, p ericardium, peritoneum, urinary bladder, cerebrospinal \202uid, or into cystic tumors off ers the potential advantage of direct access of radio- pharmaceuticals to tumor tissue with out a systemic burden. Colloids and monoclonal antibodies labeled with 32P, 90Y, or 131I ca n be used for this purpose. Monoclonal antibodies Monoclonal antibodies were considered the ultimate \200\230magic bul lets \200\231 for cancer therapy when introduced 20 years ago. The goal has been to develop antibodies that target active tumor cells speci \201cally and act as carriers of radiatio n to treat the disease. At present radio- immunotherapy has met with more problems than suc cesses and its future is uncertain. 1 ICRU Report 38 (1985) Doses and volume speci \201cation for reporting intra- cavity therapy in gynecology. International Commission on Radiation Units and Measurement. Bethesda Md, USA. 2 ICRU Report 58 (1997) Dose and volume speci \201cation for reporting interstitial therapy. International Commission on Radiation Units. Bethesda Md, USA. Further reading 1 Nag, S. (ed.) (1997) Principles and Practice of Brachytherapy Futura, New York. OHO-08(93-134) 11/16/02 11:19 AM Page 134 PRINCIPLES OF RADIATION ONCOLOGY 134 2 Dische, S. and Saunders, M.I. (2000) Modi \201ed fractionation schemes. In Oxford Textbook of Oncology (2nd edn). Oxford University Press. OHO-09(135-180) 11/16/02 11:20 AM Page 135 Chapter 9 Principles of chemotherapy Rationale for combination therapy 136 Alkylating agents 138 Anti-tumor antibiotics 142 Anti -metabolites 147 Cisplatin and derivatives 154 Topoisomerase inhibitors 159 Anti-microtubul e agents 163 Dose intensi \201cation 170 Chemo-irradiation 173 Pharmacokinetics/pharmacody namics of anti-cancer drugs 175 OHO-09(135-180) 11/16/02 11:20 AM Page 136 PRINCIPLES OF CHEMOTHERAPY 136 Rationale for combination therapy Cytotoxic chemotherapy destroys cancer cells. Currently available drugs target: (cid:2) Che mistry of nucleic acids. (cid:2) DNA or RNA production. (cid:2) Mechanics of cell division (e.g. spindle poisons). The discovery and development of cytotoxics has paralleled the unde rstanding of the chemical processes involved. The lack of selec- tivity inherent in this ap proach has limited the ability to kill cancer cells while leaving normal dividing cells uns cathed. There are only rare examples where single-agent therapy is suf \201ciently active to eradicate the clinically apparent cancer totally and lead to a durable remission or cure . Cytotoxic agents can be classi \201ed by: (cid:2) Chemical properties or mechanisms of action (alkylators) (cid:2) Source (natural pr oducts). (cid:2) Propensity to be cell cycle or phase speci \201c. The following underlie the design of a potential combination therapy: (cid:2) Each drug should have single-agent a ctivity in that tumor type. (cid:2) Each drug should have a different mechanism of activity . (cid:2) Drugs with non-overlapping toxicity patterns are preferable. (cid:2) Drugs that w ork in different parts of the cell cycle should be selected. (cid:2) Drugs should not all share the same resistance mechanisms. Combination therapy aims to increase \200\230fractional cell kill \200\231 leading to improved overall response of the tumor. Higher doses of cytotoxic drugs tend to produce increased cell kill (at least w ithin certain limits) thus it is important not to compromise on the dose of each agent (hen ce the need to select drugs with non-overlapping toxicity). Tumor growth also needs to be considered. Tumor mass is usually composed of cells that are asynchronously dividing \200\224thus com- binations of drugs that act at different points i n the cell cycle will theoretically kill more cells. \200\230Multi-drug resistance \200\231 is displayed by some tumor types, resulting from ex pression of an ef \202ux pump on the cell surface that pumps the drug out of the cell. Thi s resistance is then apparent to a set of agents known collectively as natural products. Th e combination should, therefore, not include two such agents. OHO-09(135-180) 11/16/02 11:20 AM Page 137 RATIONALE FOR COMBINATION THERAPY 137 There are other considerations in developing a combination regi- men, including: (cid:2) Schedule of administration. (cid:2) Frequency of administration. (c id:2) Possible synergistic or antagonistic interactions. (cid:2) Possible pharmacokinetic o r pharmacodynamic interactions. Novel agents in development target other aspects of maligna nt behavior, including: (cid:2) Angiogenesis. (cid:2) Autocrine and paracrine growth regula tors. (cid:2) Cell matrix interactions. The future challenge is to incorporate such drugs i nto new and existing combinations to improve patient outcomes OHO-09(135-180) 11/16/02 11:20 AM Page 138 PRINCIPLES OF CHEMOTHERAPY 138 Alkylating agents The oldest anti-cancer cytotoxic-alkylating agents \200\224are anti- neoplastic drugs becau se they bind covalently via alkyl groups to DNA. They are divided into two groups: (cid:2) Unifunctional \200\224with one reactive group. (cid:2) Bifunctional \200\224with two reacti ve groups (more ef \201cient at forming DNA cross-links). Covalent binding to DNA occurs through the formation of a reactive aziridinium group. The most common sites of alkylation are the N7 and O6 positions of guanine. Following cross -linking there is thought to be an arrest in G1-S transition followed either by DNA repair or apoptosis. Clinical use The following have been observed: (cid:2) As well as toxicity to mucus membran es, sulfur mustard and mustard gas caused lymphoid aplasia. (cid:2) Mustard gas exposure resulted in myelosuppression. (cid:2) Nitrogen mustard could b e used to treat leukemia and lymphoma. More recently, the alkylating agents have become wid ely used in can- cer chemotherapy. Resistance Resistance to alkylating agents is multifactorial and may differ between classes of alkylating agents e.g. resistance to nitrosoureas is probably mediated by increased exp ression of the enzyme O6-alkyl transferase. In addition to DNA repair, resistant cells may exhibit an increased ability to detoxify alkylating agents. Such mechanisms include increas ed: (cid:2) Glutathione. (cid:2) Metallothionein. (cid:2) Glutathione-S-transferase. Major groups of alkylating agents Nitrogen mustards The original mustard, mechlorethamine, was highly reactive with a half-li fe of minutes in biological \202uids. Other members \200\224melphalan and chlorambucil \200\224are less chemically reactive and can be adminis- OHO-09(135-180) 11/16/02 11:20 AM Page 139 ALKYLATING AGENTS 139 tered orally. Melphalan is a derivative of nitrogen mustard and the amino acid phenylalanin e. The rationale behind this was that dividing cells might take up amino acids more rapidly (and hence melphalan), thus providing some tumor selectivity. Chlorambucil is the phenylbutyric-acid derivative of nitrogen mus- tard, a well-absorbed al kylating agent with activity in both solid and hematological malignancies. CICH2CH2 N CICH2CH2 CH2 N CH2 O P H CH2 Ifosfamide O CICH2CH2 Chloroacetaldeehyde + dechloroethylated metabolites CICH2CH2 N OH CICH2CH2 CH N P O CH2 H O CH2 4-Hydroxy ifosfamide CICH2CH2 N O CICH2CH2 CH N CH2 P O H CH2 Aldoifosfamide O CH \200\223CHO CH2 Acrolein CICH2CH2 N CICH2CH2 N CICH2CH2 CICH2CH2 COOH CH2 N P O N P O H OH H O CH2 Isophosphoramide mustard Carboxyifosfamide Metabolism of ifosfamide. OHO-09(135-180) 11/16/02 11:20 AM Page 140 PRINCIPLES OF CHEMOTHERAPY 140 Oxazaphosphorines The observation that tumors overexpressed phosphoramidases led to attempt s to create an oxazaphosphorine that could be selectively cleaved by phosphoramidases withi n tumor cells, leading to the synthesis of cyclophosphamide. Subsequently, two other oxazap ho- sphorines were synthesized \200\224ifosfamide and trophosphamide. By the 1960s it becam e apparent these drugs were not activated within tumors but within the liver. It appears that the oxazaphosphorines are hydroxylated by Cytochrome P450 to the 4-hydroxy compound that is probably the transportation form of the drug. 4-hydroxy compounds undergo a series of complex intracellular degradation to form the active mustard. Cyclophosphamide is extensively used in cytotoxic chemotherapy. It is well absorbed and has nearly 100% bioavailability. Its major toxicities are: (cid:2) Myelosuppression. (cid:2) H air loss. (cid:2) Emesis. As a result of its relative lack of non-hematological toxicities, it is used in high-dose chemotherapy regimens. Ifosfamide is an isomer of cyclophosphamide. In addition to the usual pathway of 4 hydroxyl ation, a signi \201cant proportion of the ifos- famide dosage can be N-dealkylated, libera ting chloroacetaldehyde, that is thought to be responsible for some of the toxicity pro \201le. Ifosfamide nearly always causes alopecia and haemorrhagic cystitis, but this can be circumv ented by co-administration of thiol mesna which is thought to chemically combine with acrol ein, the metabolite thought to be responsible for this toxicity. Ifosfamide can also cause encephalopathy; this is more common with oral administration. Alkylalkane sulphonates Busulphan is the only member of the alkylalkane sulphonates group n ow used in clinical practice \200\224a bifunctional alkylating agent with a special role in treating chronic myeloid leukemia. It is well absorbed from the gastrointestinal tract. Th e dose-limiting toxicities are myelo- suppression and hepatic veno-occlusive disease. It ca n also cause hyperpigmentation and, rarely, pulmonary interstitial \201brosis. The nitrosoureas The discovery in the 1950s of the anti-cancer properties of N \200\231-nit roso- N-nitrosoguanidine led to synthesis of many chloroethyl nitroso- ureas. They tend to form C-G DNA intrastrand cross-links. Not all group members form cross-links e.g. methylnitrosourea and streptozocin modify DNA b y covalent bonding of a methyl group at the O6 position of guanine. BCNU is a small lipophi lic molecule and is OHO-09(135-180) 11/16/02 11:20 AM Page 141 ALKYLATING AGENTS 141 used to treat CNS tumors and as a conditioning agent in high-dose therapy. Toxicities inclu de: (cid:2) Myelosuppression. (cid:2) Veno-occlusive disease. (cid:2) Pulmonary \201brosi s. CCNU \200 is very similar to BCNU \200 , administered orally, but can also cause nephro toxicity. Aziridines Thiotepa undergoes complex metabolism. The major, perhaps active, metabolite is TEPA. The principal toxicity is hematological and it is therefore used in high-dose chemoth erapy. Tetrazines A group of molecules that undergo enzymatic or chemical conversion to liberate t he highly reactive diazonium ion, which alkylates DNA at the O6 position of guanine; as suc h they are monofunctional alkylat- ing agents. Dacarbazine is a tetrazine that requires met abolic activa- tion and is active in melanoma. Temozolomide is a tetrazine that undergoes n on-enzymatic activation and appears to be active in glioma. The future of alkylating agents A number of compounds bind by hydrogen bonding to the minor groove of DNA and then form cross-links. They exhibit sequence selectivity as to which nuc leotide sequences they bind. They may, thus, be useful in targeting certain tumors selectiv ely. Other approaches to selective tumor kill involve the use of: (cid:2) ADEPT (Antibody-D irected Enzyme Pro-drug Targeting) \200\224a tumor-speci \201c antibody linked to an enzym e that is not normally expressed in human cells is given to the patient, followed by a pro- drug that is activated by this enzyme to a cytotoxic agent. (cid:2) GDEPT (Gene-Directed Enzyme Pro-drug Targeting) \200\224similar to ADEPT but tumor is made to express the enzyme by gene therapy techniques. Many of the pro-drugs currently being used are alkylating agents. \200 CCNU: 1-(2-chloroethyl-3-cyclo-hexyl-1)- nitrosourea = lomustine \200 BCNU: 1,3-bi s(2-chloroethyl)-1-nitrosourea = carmustine. OHO-09(135-180) 11/16/02 11:20 AM Page 142 PRINCIPLES OF CHEMOTHERAPY 142 Anti-tumor antibiotics Some of most important anti-cancer drugs are products of microbial fermentation. Anthracycl ine antibiotics, doxorubicin and daunorubicin, are widely used in treating solid tumors and hematological malignan- cies respectively. Recently, liposomal formulations have entered c linical practice. Epirubicin was developed as a less cardiotoxic analog of dox- orubicin. I darubicin is an analog of daunorubicin with increased activity in AML and the only anthracy cline that can be given orally. Mitoxantrone is the most important of anthracenediones, a g roup of compounds structurally related to the anthracyclines. Two other struc- turally dist inct anti-tumor antibiotics are in use: (cid:2) Actinomycin D, active in childhood tumors. (cid:2) Mitomycin C, used both as a cytotoxic and a radiosensitizer. Anthracyclines Anthracyclines (doxorubicin, daunorubicin, epirubicin and ida- rubicin) are closely structurally related and have similar mechanisms of action and resistance, but have different patterns of clinical activity and toxicity. Pharmacology The anthracyclines have several effects, and their speci \201c mode of action is unclear. There are direct effects at the cell surface and also on signal trans- duction, speci \201 cally activation of protein kinase C-mediated cell signaling pathways. The role of these ac tions in mediating anthra- cycline cytotoxicity is unde \201ned. Their ability to undergo reduction to highly reactive compounds and generate free radicals has clinically important implications. Char- acteristic cardiotoxicity of anthracyclines ap pears due to the generation of free radicals in the heart where defense systems are less ac tive. The major target of anthracyclines is the enzyme topoisomerase II. During cell division top oisomerase II binds to DNA forming a \200\230cleav- able complex \200\231 that makes trans ient \200\230nicks \200\231 in DNA, allowing torsional strain in DNA to be released, after which strands rejoin. Anthra- cyclines bind to the cleavable complex, disrupting this proc ess, leading to DNA strand breaks and cell death. Drug resistance Some tumors are inherently resistant to anthracyclines whereas others initi ally respond but later become resistant. OHO-09(135-180) 11/16/02 11:20 AM Page 143 ANTI-tumor ANTIBIOTICS 143 The MDR1 gene codes for a P-170 glycoprotein (Pgp) that is a naturally occurring cell-surfa ce pump. Its physiological function appears to be a protective mechanism, expelling toxic s ubstances from the cell. Cell lines resistant to anthracyclines often have increased expres sion of Pgp and sensitivity can be restored by the addition of Pgp inhibitors such as verap amil or cyclosporin A. The importance of Pgp in the clinic is less clear. Though expression is increased in some human cancers before treatment or at relapse, attempts to manipulate Pgp have had limited success. It can be dif \201cult to achieve potentially effective plas ma levels of Pgp modulators without causing side-effects. Cyclosporin A and other modulator s can also in \202uence anthracycline pharmacokinetics, directly increasing exposure to th e cytotoxic. To date, modulation of Pgp has shown most promise for patients with hematologi cal malignancies. There remains a need for speci \201c, well-tolerated inhibitors of Pgp. A second ef \202ux pump associated with expression of multi-drug resistance-associated pro tein (MRP) gene has been implicated in anthracycline resistance in the lab. Reduced activity of the target enzyme topoisomerase II has also been associated with in vitro anthracycline resistance. Pharmacokinetics and metabolism After IV administration, anthracycline levels fall rapidly as it is distrib- uted and binds to tissue DNA. Subsequent metabolism and elimina- tion lea ds to a slow fall in plasma concentrations over several days. The principal route of metabolism is by reduction and subsequent hepatic elimination. The s igni \201cance of liver dysfunction in altering anthracycline kinetics has been controvers ial. Dose reductions are, however, recommended for patients with abnormal liver biochemistr y tests as they are at risk of increased toxicity. Dose reductions are not usually required for patients with impaired renal function. Structural differences affect the pharmacology and clinical use of anthracycline analogs. E pirubicin differs from doxorubicin only by the orientation of a single -OH group, but this enables glucuronida- tion in the liver and more rapid elimination. Idarubicin has a different side-chain from daunorubicin making it more lipid-soluble and al lowing oral in addition to IV administration. It is also distinctive in that the metabolite idarubicinol retains greater cytotoxic activity than metabolites of other anthracyclines. Clinical use The anthracyclines are among the most active cytotoxic agents. (cid:2) Doxorub icin and epirubicin are effective against: \200\224breast cancer. \200\224small-cell lung cancer. \200\224sarcoma. OHO-09(135-180) 11/16/02 11:20 AM Page 144 PRINCIPLES OF CHEMOTHERAPY 144 \200\224hematological cancers. \200\224pediatric malignancies. Usually used as combination agent; IV bolus or IV infusion every 3 weeks. (cid:2) Daunorubicin and idarubicin: \200\224main role in treatment of acute leukemia. \200\224IV administration (idarubicin c an be given orally). Toxicity The dose-limiting acute toxicities are: (cid:2) Myelosuppression and mucositis, bo th occuring 5 \200\22310 days after treatment. (cid:2) Alopecia occurs but is reversible. (cid:2) Extravasation injury can be severe and t here is no proven, effective treatment. Cumulative cardiotoxicity is speci \201c to anthracyclines and appears to be caused by acc umulation of free radicals in the heart. It is character- ized by dilatation of sarcoplasmi c reticulum and loss of myo \201brils. Children appear more sensitive to these effects. Cardiotoxicity typically presents with heart failure, the risk of which is dose-related. At doxorubicin doses below 450 mg/m2, the risk is less than 5%, but increases substantially a t higher doses. In most cases this threshold allows a full course of anthracycline to be gi ven without risk. Irradiation of the heart, which may occur with radiotherapy to the left b reast or chest, increases risk of cardiotoxicity, as does pre- existing cardiac disease. Cardiotoxicity appears to be related to peak drug levels as doxo- rubicin is less cardiotox ic when given as a 96-hour infusion than by bolus administration. Liposomal encapsulation o f doxorubicin also reduces cardiotoxicity. Epirubicin is less cardiotoxic than doxorubicin, the \200\230threshold \200\231 dose being 900 mg/m2. This two-fold difference in cardiotoxicity compares with 1.2-fold difference in therapeutic activity. In terms of cardio- toxocity, epirubicin has a 1.6-fold higher thera peutic index than doxorubicin, so more prolonged treatment may be possible with epirubicin before there is an unacceptable risk of cardiotoxicity. Daunorubicin and idarubicin also ha ve less effect on the myocardium than doxorubicin. Serial estimation of cardiac ejection fraction by gated isotope scan- ning or echocardiogra phy detects changes in cardiac function before becoming clinically signi \201cant. Where p atients do develop cardiac failure it often responds to standard treatment. Iron appears to generate free radicals that are associated with anthracycline treatment. The iron chelator ICRF 187 may protect against cardiotoxicity. OHO-09(135-180) 11/16/02 11:20 AM Page 145 ANTI-tumor ANTIBIOTICS 145 Mitoxantrone One of the anthracenediones, synthesized as analog of the anthra- cyclines. In common with the anthracyclines, mitoxantrone binds to DNA and interacts with topoisomerase II but appears less potent in generating free radicals. Mitoxantrone is also a substrate f or Pgp. Like doxorubicin, mitoxantrone is rapidly and highly tissue-bound. The main clinical use of mitoxantrone has been as an alternative to doxorubicin in advanced breast cancer, as it is substantially less cardiotoxic, less vesicant, and causes less alopecia. However, mitox- a ntrone is less effective than doxorubicin. It has some activity against other solid tumors, including non-Hodgkin \200\231s lymphoma and non- lymphocytic leukemia. Actinomycin D Structurally distinct from the anthracyclines, Actinomycin D binds strongly t o DNA by intercalation and inhibits synthesis of RNA and proteins. In pre-clinical models, reduced cellular uptake is associated with resistance to Actinomycin D; it also appears to be a substrate for the Pgp pump. Actinomycin D is especially active against childhood tumors and is used in combination therapy for: (cid:2) Wilms \200\231 tumor (cid:2) Rhabdomyosarcoma (cid :2) Neuroblastoma (cid:2) Ewing \200\231s sarcoma The dose-limiting toxicity is myelosuppre ssion, maximal around 10 days after treatment. Actinomycin D can also cause: (cid:2) Nausea and vomiting (cid:2) Mucositis (cid:2) Diarrhea (cid:2) Alopecia Mitomycin C (MMC) MMC is active against a range of solid tumors but is also used as a radio -sensitizer in chemo-irradiation. MMC is inactive in its natural form and requires reduction before binding to DNA by alkylat ion to form adducts. This bioreductive acti- vation may occur preferentially under the anae robic conditions that are known to exist within some solid tumors. Causes of resistance have not been well de \201ned, but as a naturally occuring compound it appears to be a substrate for Pgp. MMC is used in combination with other cytotoxics to treat breast cancer, non-small-cell lun g cancer, and GI cancer. It is used as a OHO-09(135-180) 11/16/02 11:20 AM Page 146 PRINCIPLES OF CHEMOTHERAPY 146 radio-sensitizer in the treatment of anal cancer. MMC is effective in reducing recurrence o f super \201cial bladder cancer where it is given intravesically, effectively eliminating systemic exposure to the drug. The most important toxicity of MMC is myelosuppression, especially thrombocytopaenia, which is delayed and can be cumulative. Accordingly, MMC is given systemically every 6 weeks in contrast to the 3-weekly schedules usually used for other anti-tumor antibiotics. Haemolyti c-uraemic syndrome, pulmonary \201brosis and, cardiac complications are all uncommon, esp ecially at low cumulative doses, but are potentially fatal. Other toxicities such as nausea and vomiting, alopecia, and stomatitis are usually mild, but extravasation can be serious. OHO-09(135-180) 11/16/02 11:20 AM Page 147 ANTI-METABOLITES 147 Anti-metabolites Anti-metabolites interfere with normal cellular metabolism of nucleic acids; they act with cell-cycle S-phase speci \201city. An early compound induced the \201rst remissions in c hildhood ALL, and they now include some of the most widely prescribed cytotoxic agents, who se indica- tions are not con \201ned to treating malignancies. Anti-folates Understanding anti-metabolite action necessitates knowledge of folate biochemi stry. The enzyme thymidylate synthase (TS) acts as rate-limit- ing step in the synthesis of thymidylate, converting dUMP into dTTP by transferring a methyl group from CH2-FH4. The su pply of reduced folate is maintained by the enzyme dihydrofolate reductase (DHFR). Methotrexate (MTX) Widely used, MTX is effective in: (cid:2) Breast cancer (cid:2) Osteogen ic sarcoma RNA 5-CHO-FH4 (Leucovorin) 10-CHO-FH4 dUMP FUTP CH2-FH4 Tegafur DHFR MTX FduMP 5-FU TS TS inhibitors DHPD Capecitabine dTMP FH2 Elimination dTTP DNA MTX Cytotoxics (MTX \200\224methotrexate; 5-FU \200\2235 \200\223fluorouracil; TS inhibitors i nclude ralitrexed) dTTP Normal metabolites Indicates enzyme inhibition Enzyme (TS \200\224thymidylate synthase; DHPD \200\224dihydropyrimidine dehydrogenase; DHFR \200\224dihydrofolate reductase) DHPD Main sites of action of anti-metabolites. OHO-09(135-180) 11/16/02 11:20 AM Page 148 PRINCIPLES OF CHEMOTHERAPY 148 (cid:2) Bladder carcinoma (cid:2) Gastric cancer (cid:2) Choriocarcinoma (cid:2) Head and n eck cancers (cid:2) Leukemia (cid:2) Lymphoma Also used for psoriasis and rheumatoid arthri tis, MTX is actively transported into cells where it is polyglutamated, binds tightly to an d inhibits DHFR. It also: Inhibits other enzymes e.g. TS Impairs DNA repair (cid:2) (cid:2) (cid:2) Causes accumulation of dihydrofolate polyglutamates (that in turn inhibit folate-dependent enzymes) Pharmacology MTX is well absorbed orally below 25 mg/m2, but is usually adminis- tered IV, except in maintenance regimens and treatment of benign connective tissue diseases. It is el iminated in three phases, the second and third phases each taking 10 \200\22324 hours; elim ination is considerably lengthened by renal dysfunction. There is some hepatic metabolism t o the active drug 7-OH-MTX and approximately 10% of the drug is cleared by biliary excretio n. Dose adjustments are not usually neces- sary with hepatic dysfunction. Signi \201cant t hird-space effects occur in the presence of \202uid collections (e.g. ascites, pleural ef fusions) and can increase toxicity through reduced clearance. MTX excretion can also be inh ibited by: (cid:2) Probenicid (cid:2) Penicillins (and cephalosporins) (cid:2) Non-steroida l anti-in \202ammatory agents Evidence shows synergy with 5- \202uorouracil (5-FU), which is maximal if given 24 hours before the 5-FU. Resistance occurs through: Impaired uptake (cid:2) (cid:2) Gene ampli \201cation of DHFR (cid:2) Decreased polyglutamation Toxicities include : (cid:2) Mucositis (cid:2) Myelosuppression (cid:2) Nephrotoxicity (cid:2) Acute and chron ic hepatotoxicity (cid:2) Self-limiting pneumonitis (cid:2) Arachnoiditis and chronic demye lination (when given intrathecally) OHO-09(135-180) 11/16/02 11:20 AM Page 149 ANTI-METABOLITES 149 High-dose MTX has been used in sarcomas, has the advantage of some CSF penetration, and may overcome resistance. Folinic acid can be used to \200\230rescue \200\231 normal cells fro m undue toxicity, usually given 24 hours after MTX. Methotrexate analogs Aminopterin has been surpassed by MTX, though no trial comparing them has been done. Edatrexate is an analog, with better uptake characteristics and is active in breast, non-small-cell lung, and head and neck cancer. Trimetrexate is lipophilic, does no t require poly- glutamation or the folate uptake pathway, and is currently under evaluation in clinical trials. Thymidylate synthase (TS) inhibitors New agents have been developed that directly inhibit T S (in contrast to indirect inhibitors e.g. 5-FU and MTX) and interact with the folate- bind ing site of TS. Glutamated members utilize the folate-carrier system for entry into cells a nd are polyglutamated, enhancing their ef \201cacy and intracellular retention. Non-glutam ated compounds are lipophilic and may have different effects. Raltitrexed (Tomudex) is a glutamated compound that causes pro- longed inhibition of TS. It has triphasic elimination, with a rapid ini- tial fall in concentration but very prolonged \201nal phase. 50% of the drug is renally excreted unchanged. It is active in breast and colorectal cancer with toxicities including: (cid:2) Myelosuppression (cid:2) Diarrhea (ci d:2) Transaminitis It is usually administered IV once every 3 weeks. Fluropyrimidines These pro-drugs are intracellularly activated and their products inhibit p yrimidine synthesis.