Tutorial Article - AAEP · Tutorial Article The use of cytotoxic drugs in equine practice T. S....

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Tutorial Article The use of cytotoxic drugs in equine practice T. S. MAIR* AND C. G. COUTO Bell Equine Veterinary Clinic, Mereworth, Maidstone, Kent ME18 5GS, UK; College of Veterinary Medicine, The Ohio State University, 1900 Coffey Road, Columbus, Ohio 43210, USA. Keywords: horse; neoplasia; cancer; cytotoxic medicine; ageing 188 EQUINE VETERINARY EDUCATION / AE / JUNE 2006 Introduction Neoplastic diseases are a major cause of morbidity and mortality among domestic animals, just as they are in man. In the horse, the skin is the organ most commonly affected by neoplasia and the sarcoid is the most common neoplasm occurring at this site (Sundberg et al. 1977; Goodrich et al. 1998). However, other neoplasms affecting all parts of the body can and do occur, albeit at a lower frequency than tumours of the skin. With some exceptions, neoplasms tend to be associated with ageing. Ageing per se is not generally considered a direct factor in the induction of tumours, but presumably allows increased opportunity to develop neoplasia, including cumulative exposure to environmental carcinogens (Cockerall and Cooper 2002). This is probably one reason why neoplasms are seen more commonly in dogs and cats than in other domestic animals, because these species are allowed to live out their natural life span. As geriatric populations of equids increase, so it is likely that more cases of neoplasia will be encountered in the horse population (Savage 1998a). However, not all cases of neoplasia occur in older horses, and some specific tumours are commonly seen in young animals, including lymphoma (lymphosarcoma) (Rebhun and Bertone 1984; Mair et al. 1985; Mair and Hillyer 1992) and hepatocellular carcinoma (Roby et al. 1990; East and Savage 1998). Pathogenesis, cancer biology and clinical staging Over the past 40 to 50 years there has been significant progress in the basic understanding of the pathogenesis of neoplasia (also known as oncogenesis or carcinogenesis). Although details of the pathogenesis of neoplasia are beyond the scope of this article, it is likely that future advances in the diagnosis, prognosis, and treatment of neoplasia in the horse and other species will be closely tied to advancements in the understanding of the pathogenesis and biology of neoplasia. It is common to refer to a neoplastic cell as a transformed cell and the associated event as transformation. At this time, there is no single, unified concept of the biological mechanisms that underlie transformation and the development of neoplastic disease, and the reader is referred to more detailed texts that describe the major theories about pathogenesis of neoplasia (Ross 1998; Cockerell and Cooper 2002; Kelsey et al. 2002). It is generally believed that most neoplasms are monoclonal in origin, i.e. they arise from genetic mutations within a single affected cell, although in certain tumours in some species (e.g. equine sarcoid and feline sarcoma) a polyclonal mechanism may be involved. Over subsequent cell divisions in the neoplasm, heterogeneity can develop through the accumulation of further abnormalities (Kelsey et al. 2002). The genes most commonly affected are those controlling cell cycle check points, DNA repair and DNA damage recognition, apoptosis, differentiation and growth signalling. Proliferation may continue at the expense of differentiation, which together with the failure of apoptosis (apoptosis is also known as ‘programmed cell death’) leads to tumour formation with the accumulation of abnormal cells (Rudin and Thompson 1997). In general, benign tumours grow slowly, and malignant tumours grow rapidly. The growth rate of an individual neoplasm is determined by such factors as the length of the mitotic cycle, proportion of cells in mitosis and rate of cell loss, amongst others. The spread of neoplastic cells by metastasis is the most serious consequence of malignancy. More cancer treatment failures and deaths in man result from the effects of metastasis than of the primary tumour (Fidler 2000; Cockerall and Cooper 2002). In many cases, metastatic dissemination of a malignant neoplasm will have occurred before it can be detected clinically. Microscopic spread, especially via blood vessels and lymphatics, is the reason why surgical removal of a primary malignant neoplasm is likely to fail to be curative in approximately 50% of patients. Grading and staging of neoplasms are commonly used in many human cancers as a means of quantifying the behaviour of the neoplasm in an individual patient. Grading represents an attempt to quantify the degree of malignancy at the *Author to whom correspondence should be addressed.

Transcript of Tutorial Article - AAEP · Tutorial Article The use of cytotoxic drugs in equine practice T. S....

Page 1: Tutorial Article - AAEP · Tutorial Article The use of cytotoxic drugs in equine practice T. S. MAIR* AND C. G. COUTO† Bell Equine Veterinary Clinic, Mereworth, Maidstone, Kent

Tutorial ArticleThe use of cytotoxic drugs in equine practiceT. S. MAIR* AND C. G. COUTO†

Bell Equine Veterinary Clinic, Mereworth, Maidstone, Kent ME18 5GS, UK; †College of Veterinary Medicine, TheOhio State University, 1900 Coffey Road, Columbus, Ohio 43210, USA.

Keywords: horse; neoplasia; cancer; cytotoxic medicine; ageing

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Introduction

Neoplastic diseases are a major cause of morbidity andmortality among domestic animals, just as they are in man. Inthe horse, the skin is the organ most commonly affected byneoplasia and the sarcoid is the most common neoplasmoccurring at this site (Sundberg et al. 1977; Goodrich et al.1998). However, other neoplasms affecting all parts of thebody can and do occur, albeit at a lower frequency thantumours of the skin. With some exceptions, neoplasms tend tobe associated with ageing. Ageing per se is not generallyconsidered a direct factor in the induction of tumours, butpresumably allows increased opportunity to developneoplasia, including cumulative exposure to environmentalcarcinogens (Cockerall and Cooper 2002). This is probably onereason why neoplasms are seen more commonly in dogs andcats than in other domestic animals, because these species areallowed to live out their natural life span. As geriatricpopulations of equids increase, so it is likely that more cases ofneoplasia will be encountered in the horse population (Savage1998a). However, not all cases of neoplasia occur in olderhorses, and some specific tumours are commonly seen inyoung animals, including lymphoma (lymphosarcoma)(Rebhun and Bertone 1984; Mair et al. 1985; Mair and Hillyer1992) and hepatocellular carcinoma (Roby et al. 1990; Eastand Savage 1998).

Pathogenesis, cancer biology and clinicalstaging

Over the past 40 to 50 years there has been significantprogress in the basic understanding of the pathogenesis ofneoplasia (also known as oncogenesis or carcinogenesis).Although details of the pathogenesis of neoplasia are beyondthe scope of this article, it is likely that future advances in thediagnosis, prognosis, and treatment of neoplasia in the horseand other species will be closely tied to advancements in theunderstanding of the pathogenesis and biology of neoplasia.

It is common to refer to a neoplastic cell as a transformed celland the associated event as transformation. At this time, thereis no single, unified concept of the biological mechanisms thatunderlie transformation and the development of neoplasticdisease, and the reader is referred to more detailed texts thatdescribe the major theories about pathogenesis of neoplasia(Ross 1998; Cockerell and Cooper 2002; Kelsey et al. 2002). Itis generally believed that most neoplasms are monoclonal inorigin, i.e. they arise from genetic mutations within a singleaffected cell, although in certain tumours in some species (e.g. equine sarcoid and feline sarcoma) a polyclonalmechanism may be involved. Over subsequent cell divisions inthe neoplasm, heterogeneity can develop through theaccumulation of further abnormalities (Kelsey et al. 2002). Thegenes most commonly affected are those controlling cell cyclecheck points, DNA repair and DNA damage recognition,apoptosis, differentiation and growth signalling. Proliferationmay continue at the expense of differentiation, which togetherwith the failure of apoptosis (apoptosis is also known as‘programmed cell death’) leads to tumour formation with theaccumulation of abnormal cells (Rudin and Thompson 1997).In general, benign tumours grow slowly, and malignanttumours grow rapidly. The growth rate of an individualneoplasm is determined by such factors as the length of themitotic cycle, proportion of cells in mitosis and rate of cell loss,amongst others.

The spread of neoplastic cells by metastasis is the mostserious consequence of malignancy. More cancer treatmentfailures and deaths in man result from the effects of metastasisthan of the primary tumour (Fidler 2000; Cockerall andCooper 2002). In many cases, metastatic dissemination of amalignant neoplasm will have occurred before it can bedetected clinically. Microscopic spread, especially via bloodvessels and lymphatics, is the reason why surgical removal ofa primary malignant neoplasm is likely to fail to be curative inapproximately 50% of patients.

Grading and staging of neoplasms are commonly used inmany human cancers as a means of quantifying the behaviourof the neoplasm in an individual patient. Grading represents anattempt to quantify the degree of malignancy at the*Author to whom correspondence should be addressed.

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microscopic level. Staging represents an attempt to classify theneoplasm according to its progression or extent, with the sizeof the primary lesion, the presence of lymph nodeinvolvement, and the presence of metastasis being taken intoaccount (Cockerall and Cooper 2002). Such grading andstaging systems are helpful not only in predicting outcome,but also for guiding treatment. For example, clinical staging ofcanine lymphoma correlates with prognosis, and can be usedto define the most effective chemotherapeutic regimen(Madewell 1999). In canine lymphoma, staging is based uponthe results of a standard set of tests, including a physicalexamination, complete blood count, biochemical profile,urinalysis, thoracic and abdominal radiographs, abdominalultrasound, lymph node and bone marrow aspiration cytology,and lymph nodebiopsy (Rosenthal 2001).

Unfortunately such systems have not been described orevaluated in the horse, mainly due to size of the patients, smallnumbers and difficulties in obtaining follow-up. The sheer sizeof the equine patient makes clinical staging difficult to perform.The identification of abdominal tumours or metastases in thehorse can be especially difficult, and often the neoplasia willhave reached an advanced stage by the time clinical signs occurand a diagnosis is achievable (East and Savage 1998). Similarly,mediastinal tumours (primarily lymphoma) may be present for aconsiderable length of time before clinical signs becomeapparent, and their identification can be difficult using standardtechniques of ultrasonography and radiography because ofanatomical factors (Mair et al. 1985).

Despite these problems, attempts to implement stagingsystems should be encouraged in the most common equineneoplasms, and would probably aid in the development ofchemotherapeutic and other approaches to treatment. Forexample, clinical staging of squamous cell carcinoma of thepenis and prepuce of male horses has been suggested as beingpotentially valuable in defining prognosis and aiding treatmentchoices (Mair et al. 2000).

Principles of chemotherapy

A variety of different treatment modalities are used for treatingneoplasia. Surgery (including traditional ‘sharp’ surgery, lasertherapy and cryosurgery) remains the most effective form oftreatment for most solid tumours of skin, soft tissues and bone,but early treatment and aggressive resection are necessary incases of malignant disease. The use of chemotherapy followingsurgical management of a primary mass is frequently used inman as a way of managing malignant disease.

Despite the increasing use of chemotherapy in the treatmentof cancer in man and small animals, the use of cytotoxic drugs inhorses has found only limited application. This is partly becauseof the high costs involved with treating horses with such drugs,but also because of perceived problems with adverse drugeffects (see below). Local chemotherapy, including topical andintratumoral chemotherapy, has been used in cutaneousneoplasms as a way of maximising the exposure of the tumourto the drug, while avoiding systemic toxicity. The high incidenceof cutaneous neoplasia in the horse makes topical/local therapy

a logical approach. The major indication for systemic use ofcytotoxic drugs in the horse is in the treatment oflymphoproliferative diseases such as lymphoma. Despite thelimitations of chemotherapy in horses, it is likely that as peoplebecome more knowledgeable about available treatments forcancer in man, they will demand more sophisticated treatmentoptions for their animals, including horses. Provided they have anunderstanding of the treatment goals and costs, many ownersare appreciative of honest attempts to control neoplastic disease(Chun 2001).

In general, chemotherapeutic agents damage DNA (andRNA) and result in cellular death, apoptosis or inability toundergo mitosis. Some may cause frank necrosis. For many ofthese drugs there is a fairly narrow therapeutic window betweeneffective systemic treatment and toxicity to normal tissues(therapeutic index). The drugs are not cancer-specific, but act onneoplastic cells preferentially because of their increased cellularproliferation rates. However, the proliferation rate in neoplastictissues is usually not that much greater than normal tissues, andtoxic effects in normal tissues are therefore common in humansreceiving systemic chemotherapy. For this reason, the dose andschedule of chemotherapy is limited by normal tissue tolerance,especially in those tissues that proliferate rapidly such as thebone marrow and gastrointestinal tract mucosa. Other tissuescan also be affected depending on the individual tissue affinity of

TABLE 1: Categories and examples of commonly usedchemotherapeutic drugs in veterinary medicine

Major mechanismsCategory of action Drug examples

Alkylating agents Damage DNA and Cyclophosphamideinterfere with cell Chlorambucilreplication Melphalan

LomustineCisplatinCarboplatin

Antitumour antibiotics Damage DNA and Doxorubicininterfere with Mitoxantronenucleic acid Actinomycin Dsynthesis Bleomycin

Plant alkaloids Distrupt mitosis in Vincristinethe metaphase Vinblastine

Hormones Immunosuppression Prednisoloneand anti-tumour Prednisoneeffects Dexamethasone

Antimetabolites Interfere with L-asparaginaseDNA and RNA Methotrexatesynthesis by Cytosine arabinosideinteration with 5-fluorouracilenzymes

Taxanes Interfere with Taxolcell division

Miscellaneous Enzymatic Asparaginasedestruction ofamino acids

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the particular chemotherapeutic agent. Although many of thechemotherapeutic agents used in veterinary medicine are thesame as those used in human medicine, veterinary applicationstend to use lower doses and less aggressive protocols, and soadverse effects are less of a problem. In veterinary patientsquality of life is generally as important as (or perhaps, moreimportant than) disease control (Chun 2001).

Many tumours will develop resistance to single-agentchemotherapy. For this reason, intermittent combinationchemotherapy was developed. With such regimes, several drugsare combined together or sequentially, chosen on the basis ofdiffering mechanisms of action and nonoverlapping toxicities(Kelsey et al. 2002). The drug combination is given over a periodof a few days, followed by a rest period of a few weeks, duringwhich time the normal tissues have the opportunity forregrowth. If the normal cells are more proficient at DNA repairthan the cancer cells, then it may be possible to deplete thetumour while allowing restoration of normal tissues betweenchemotherapy cycles.

Chemotherapeutic drugs

Categories of commonly-used chemotherapeutic agents andexamples of drugs are shown in Table 1. A number of thecytotoxic agents cause direct damage to DNA. Alkylating agentssuch as cyclophosphamide act by covalently linking alkyl groupsand their major effect is to cross-link DNA strands, therebyinterfering with DNA synthesis and causing strand breaks (Kelseyet al. 2002). These were among the earliest of the cytotoxicdrugs developed for use in man, but they are still widely used incancer treatment. Platinum compounds, including cisplatin,cause interstrand cross-links of DNA and are often regarded asnon-classical alkylating agents. As with other heavy metalcompounds, they can have toxic effects on the kidneys andperipheral nerves. The antimetabolites, such as 5-fluorouracil (5-FU), cytosine arabinoside and methotrexate, are structuralanalogues of naturally occurring metabolites; they interfere withthe normal synthesis of nucleic acids by falsely substitutingpurines and pyrimidines in metabolic pathways. The mostcommonly used drug of this class in the horse, 5-FU, consists ofa uracil molecule with a substituted thymidylate synthetasewhich is essential for pyrimidine synthesis.

The cytotoxic antibiotics, such as doxorubicin, act byintercalating adjoining nucleotide pairs on the same strand ofDNA and by inhibiting DNA repair. The vinca alkaloids, includingvincristine and vinblastine, act by binding to tubulin andinhibiting mirotubule formation (part of the cytoskeleton of thecell), thus disrupting mitosis. The taxanes also bind to tubulin andprevent their assembly into microtubules.

Topical chemotherapy for cutaneous neoplasms

Topical and intralesional chemotherapy enable highconcentrations of cytotoxic drugs to be maintained withincutaneous and other superficial neoplasms over a long periodwhile sparing adjacent normal skin (Theon 1998), thusminimising toxicity. Immunotherapeutic agents can also besuccessfully used by these application routes.

Topical treatment with 5-FU has been used successfully forthe treatment of squamous cell carcinoma of the externalgenitalia (Fortier and MacHarg 1994) and other skin sites(Paterson 1997). The treatment involves the repeated applicationof the drug to the tumour surface. In addition to having cytotoxicactivity, 5-FU acts as a contact allergen and induces a delayedhypersensitivity reaction that causes the superficial cell layers toslough (Mansell et al. 1975). Although 5-FU destroys thesuperficial layers of the tumour, the precise depth of action isimpossible to predict. Normal skin is less sensitive to the effectsof the drug because of the lower proliferation rate of skin cellsand the barrier effect of keratin that prevents the drug frompenetrating to the deeper epithelial layers. Since the topicallyapplied cream does not penetrate deeply, it is most useful forsuperficial tumours (including squamous cell carcinoma) lessthan 2 mm thick, but not for the treatment of nodular or deeplyinvasive tumours (Theon 1998). It can be used as an adjunct tosurgery for the treatment of squamous cell carcinoma aftersurgical debulking leaving a tumour bed less than 3 mm thick.Treatment should be started as soon as possible after surgerysince the drug does not affect wound healing and will notpenetrate through a healed skin wound. Treatment is generallyapplied daily for 3–4 months, or longer, until complete tumourregression has occurred. Total regression of the lesion should beconfirmed (preferably by biopsy) before treatment isdiscontinued, because accelerated regrowth can occur if thelesion is not completely eliminated (Theon 1998). The number oftreatments required can be reduced if the area can be covered bya waterproof dressing. For the treatment of squamous cellcarcinoma of the penis, the prepuce acts as a dressing and thetreatments can be reduced to applications at two week intervals(Theon 1998).

Ocular (conjunctival and corneal) squamous cell carcinomacan also be treated using topical 5-FU, either alone or incombination with other treatments. Ophthalmic dropscontinaing 5-FU have been used in man (Yeatts et al. 2000; Foyet al. 2002), but there is very limited information about theefficacy of this therapy in horses. This drug has also been used totreat equine sarcoids (Figs 1a and b), often in combination withother therapies such as surgical debulking. Sarcoids arecommon, locally invasive, non-metastatic, fibroblastic tumoursthat can affect any part of the body, but especially the ventralabdomen, limbs and head (Goodrich et al. 1998; Foy et al.2002). To aid the penetration of the 5-FU through the superficialepidermis, the drug can be mixed with podophyllin, an irritantcathartic (McConaghy et al. 1994; Piscopo 1999). Topical 5-FUneeds to be applied daily for 30–90 days. Alternatively, it can beapplied less frequently under a bandage (Roberts 1970). It ismost useful for single, small occult or verrucose sarcoids, or forlarger areas of occult or verrucose sarcoids that are not amenableto any other form of treatment. The treated area can show amarked inflammatory reaction, but scarring is usually minimal(Pascoe and Knottenbelt 1999).

Heavy metal compounds, including inorganic arsenic,antimony and mercury salts, cause tissue necrosis, and can beused topically to treat sarcoids (Pascoe and Knottenbelt 1999).A topical ointment (AW-3-LUDES)1 containing a variety of

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heavy metals with 5-FU and thiouracil has been used in the UKfor the treatment of sarcoids for several years (Knottenbelt andWalker 1994; Knottenbelt and Kelly 2000; Newton 2000). Theointment is applied on successive or alternate days for 3–5 treatments, depending on the size, number and nature ofthe sarcoids, and the strength of the ointment used. Aninflammatory reaction is usually seen after one or moretreatments. In some cases, a painful and marked oedematousreaction occurs, necessitating anti-inflammatory treatment. Afull response is usually seen in 5–10 weeks, characterised bynecrosis and sloughing of the sarcoid tissue. A granulatedwound bed usually remains once the sarcoid tissue sloughs. Aresolution rate of over 80% has been reported in cases wheresarcoids are being treated with topical AW-3-LUDES for thefirst time (Knottenbelt and Walker 1994) (Figs 2a,b and 3a,b).The success rate decreases by 30–40% for each previousunsuccessful treatment. A topical paste (Xxterra)2 containingzinc chloride and plant alkaloids derived from the bloodrootplant has recently become available in the USA, and there isanecdotal evidence that it can be successful in eliminatingsmall, localised sarcoids (Palmer 2002). A rapid response to theextract is usually seen, with inflammation followed by pusaccumulation under the tumour base and sloughing of thelesion in 7–10 days; the surrounding healthy skin is left intact.The reaction appears to be similar to that seen in graftrejection. There is anecdotal evidence of effect on sarcoidsremote from those being treated in some cases. Anotherbloodroot extract (Animex)3 containing Sanguinariacanadensis, puccoon, gromwell, distilled water and traceminerals has also recently been used to treat various skinlesions including sarcoids (Foy et al. 2002). The ointment is saidto penetrate the lesion, killing the affected cells while leavingthe surrounding healthy tissue intact. In the UK, a topicalointment of undisclosed composition is available commercially(Camrosa)4. There is anecdotal evidence of efficacy of thissubstance in the treatment of all types of sarcoid.

Unfortunately, the results of topical therapies using theabove mentioned drugs are difficult to quantify. Althoughthere are many anecdotal reports indicating good efficacy,

there appear to be few published studies, and it is not possibleto compare the short- and long-term results of thesetreatments with other therapies.

Intralesional chemotherapy for cutaneousneoplasms

Intralesional injection of cytotoxic drugs into the tumour andtumour bed allows higher intratumoral drug concentrations tobe achieved and permits accurate placement of the drugwithin the tumour. The beneficial effects of this route ofadministration are enhanced if drug carriers that prolong thepersistence of the drug in the tissue are used. Viscous fluidpreparations such as sesame seed oil or almond oil arefrequently used to achieve this effect (Theon et al. 1993). Theviscosity of the drug mixture can be altered, and vasoactiveagents included that will alter drug reabsorption, dependingon the individual characteristics of the tumour being treated.Cisplatin is the drug most widely used in this manner,although carboplatin, 5-FU and bleomycin are also suitable forintralesional use.

Fig 2: Treatment of a fibroblastic sarcoid on the cannon regionwith topical AW-3-LUDES cream. a) Before treatment and b) 2 months after treatment.

a) b)

Fig 3: Treatment of a fibroblastic sarcoid on the medial thighwith topical AW-3-LUDES cream. a) Before treatment and b) 2 months after treatment.

a) b)

Fig 1: Successful treatment of a superficial verrucose sarcoid onthe head with 5 fluorouracil ointment. a) Before treatment andb) one year later.

a) b)

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Intralesional cisplatin therapy has been found to beeffective in the treatment of a variety of solid tumoursincluding squamous cell carcinoma (Fig 4), sarcoid, soft tissuesarcomas, lymphoma, plasmacytoma and hamartoma (Theon1998). For melanomas, it has been suggested that moreeffective treatment is achieved when a vasoactive agent suchas adrenaline is added to the formulation (Theon 1997).Intralesional cisplatin can be effective as the sole treatment fortumours measuring up to 3–5 cm in diameter; for largertumours (>20 cm3), the treatment should usually be combinedwith surgery (Theon et al. 1994). Emulsions made up from 1 ml of an aqueous solution of 10 mg cisplatin/ml and 2 mlsesame oil are frequently used at a dose rate of 1 mgcisplatin/cm3 of tumour (Goodrich and Semevolos 2000). Theemulsion is created by attaching 2 luer-lock syringes (one withthe aqueous cisplatin solution and one with the sesame oil) toa 3-way valve; the valve is closed towards the exit (i.e. needleport) and the solution and oil are mixed by alternatively

pushing the plunger of each syringe until a milky whitesuspension develops. The target volume of tissue to beinjected should include all visible tumour plus a margin ofnormal tissue of 1–2 cm (Theon 1997). This formulation ofcisplatin (3.3 mg of cisplatin/ml of emulsion) is relatively stableand will not separate into two phases over a 3 hour period(Theon et al. 1993). The tumour and surrounding tissuemargin are infiltrated with the emulsion through a 20–23-gauge needle attached to a luer-lock syringe; the laststep is important, since the high viscosity of the emulsionfrequently causes for the needle to get detached from thesyringe (spilling its contents on the patient or the operator) ifa luer-lock mechanism is not used. To administer the drug, theneedle is inserted into the tissue to the desired depth and thedrug is injected as the needle is withdrawn. The desiredvolume of emulsion is injected via multiple sites using aparallel-row technique or field-block technique. The rows of

Fig 4: Squamous cell carcinoma of the vulva in an aged mare. a) Before treatment and b) after 4 treatments withintralesional cisplatin (administered at approximately 3-weekintervals). The tumour mass is significantly reduced in size,although complete resolution was not achieved in this marefollowing further treatments.

Fig 5: Horse receiving intravenous chemotherapy for lymphoma(note vinyl gloves are being worn by the operators, but theseare now known to not be protective).

b)a)

Fig 6a: Mare in the early stages of chemotherapy formulticentric lymphoma using the COP (cyclophosphamide,vincristine and prednisolone) protocol. At this time the marewas pregnant (second trimester), had been receivingchemotherapy for 7 weeks and was in complete remission.

Fig 6b: Mare shown in Figure 6a with foal after successfulchemotherapy for multicentric lymphoma. Chemotherapy wasinitiated in the second trimester of pregnancy and was notassociated with adverse effects.

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injections should be kept 0.6–0.8 cm apart (Theon 1997).Injection into multiple tissue planes may be required (Goodrichand Semevolos 2000). Repeated injections every 2 weeks arecontinued until all clinical evidence of neoplasia hasdisappeared. Local wound toxicosis is minimal andperioperative cisplatin treatment does not appear to affectwound healing. Alternative formulations of cisplatin havebeen described. An emulsion of almond oil and cisplatin witha final concentration of 1 mg cisplatin/ml, injected every 5–7 days, resulted in complete resolution in 6 of 18 cases ofperiorbital sarcoid in one report, and improved but did notresolve a further 10 of 18 cases (Knottenbelt and Kelly 2000).A mixture of aqueous cisplatin (5 ml; 1 mg/ml) and 1 ml ofepinephrine (adrenalin 1:1000) is described by Doyle (1998);the solution is administered weekly.

Intralesional cisplatin treatment generally has goodcosmetic results, and can be used at sites such as the eyelid,pinna, muzzle, penis and base of tail (Theon 1998;Knottenbelt and Kelly 2000). A 2-year local control rate ofapproximately 90% has been reported for sarcoids and 70–90% for squamous cell carcinoma (Theon et al. 1993,1994; 1998; Goodrich and Semevolos 2000).

Intralesional 5-FU has been used most commonly in thetreatment of squamous cell carcinoma. In one study 5-FU was found be ineffective as a single intralesional agentfor the treatment of squamous cell carcinoma (Orenberg et al.1992), although it can be effective when combined withsurgery or used as multiple treatments. A solution of 5-FU(500 mg in 10 ml) can be mixed with 3 ml of epinephrine (1 mg/ml) and administered intralesionally (50–60 mg/cm3)(Doyle 1998). It may also be injected together with cisplatin.At The Ohio State University we have used a 5-FU/ sesame oilemulsion for intralesional injection of squamous cellcarcinomas with a high degree of success. We typically mix250 mg of 5-FU (i.e. 5 ml) with 3–5 ml sesame oil in a 12 mlluer-lock syringe and administer every 2 weeks for 3–5 treatments as described above. Adverse effects areminimal and over 75% of the tumors experience complete orpartial remission.

A variety of other agents administered by intralesionalinjection have been used in the treatment of cutaneousneoplasms in horses. These include chemicals such asformalin, calcium chloride, zinc chloride and copper sulphate.Although there are anecdotal reports of their efficacy, nopublished studies exist to support the use of such substances.

Systemic chemotherapy

Systemic chemotherapy has been used and reported relativelyinfrequently in adult horses. There are, however, a limitednumber of reports of chemotherapeutic treatment of equinelymphoma (lymphosarcoma) (Fig 5). Lymphoma is thecommonest of the lymphoproliferative diseases in horses. It isalso the commonest malignant neoplasm seen in this species(Rebhun and Bertone 1984) and carries a grave prognosis. Asthe disease is most commonly multicentric or generalised,surgical resection of lymphomatous masses is unlikely to bebeneficial. However, in cases of solitary lymphoma with nospread, local surgical resection or radiotherapy can bebeneficial or curative (Coumbe 1994). Small intestinal andlarge colon resection have proved curative in a limitednumber of cases of solitary intestinal lymphoma masses(Dabareiner et al. 1996).

Chemotherapeutic protocols have been used in cases ofgeneralised lymphoma with some limited success (Couto1994; Savage 1998b; Johnson 1998). Most protocols haveincluded combinations of vincristine, cyclophosphamide,cytosine arabinoside, chlorambucil, doxorubicin, dactinomycinand prednisolone. The dosages are generally extrapolatedfrom small animal and human oncology, and are calculated ona square metre basis using the following formula (Couto1994; Doyle 1998):

An approximately 50% remission is usually achieved and isseen within 2–4 Weeks (Figs 6a and b). Dosages for thecommonly used antineoplastic agents are summarised in Table 2.

The Ohio State University currently uses the CAP protocol,a combination of cytosine arabinoside at an average dose of 1–1.2 g (total dose), subcut. or i.m., once every 1–2 weeks;cyclophosphamide at a dose of 1 g (total dose) i.v., every 2 weeks (alternating with cytosine arabinoside); andprednisolone at a dose of 1 mg/kg, per os, every other day.Vincristine (2.5 mg, i.v.) is added on the weeks when thecytosine arabinoside is administered if there is no response.These are starting doses; the total doses can be increased by20–30% without expecting complications. With remission,the starting doses are maintained for 2–3 months and then

Body surface area (m2) = weight (g2/3) x 10.5 104

TABLE 2: Dosages of chemotherapeutic agents used systemically in equine neoplasia

Drug Dose rate Route Frequency Typical doses

Prednisolone 1 mg/kg per os Every other dayVincristine 0.5 mg/m2 i.v. Weekly 2.5–3.0 mg/horse/doseCytosine arabinoside 200–300 mg/m2 subcut., i.m., i.v. Every 1–2 weeks 1–1.5 g/horse/doseCyclophosphamide 200 mg/m2 i.v. Every 1–2 weeks 1 g/horse/doseDoxorubicin 20–30 mg/m2 i.v. Every 2–3 weeks 2.5–3.0 mg/horse/doseDactinomycin 0.5–0.75 mg/m2 i.v. Every 3 weeks 2.5–3.0 mg/horse/doseChlorambucil 20 mg/m2 per os Every 2 weeks 100 mg/horse/doseL-asparaginase 10,000–40,000 i.u./m2 i.m. Every 2–3 weeks 50,000–200,000 i.u./horse/dose

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the horse is switched onto a maintenance protocol. The firstcycle of maintenance therapy increases the treatmentinterval for each antineoplastic agent by one week (exceptprednisolone which is kept at the same frequency but witha reducing dose). If the horse is still in remission after 2–3months of the first cycle, the second cycle is begun byadding a further week to the treatment intervals of eachdrug. Most horses in remission are treated for a total of 6–8months (Schneider 1997). Approximately 50% ofmulticentric lymphoma patients can be brought intoremission for several months to a year using this protocol.Recurrence is likely following withdrawal of the treatment.Horses receiving systemic chemotherapy should bemonitored for signs of gastrointestinal, renal or neurologicaltoxicity. Weekly haematological evaluations, as well asoccasional serum biochemical profiles, are advised.Myelosuppression and gastrointestinal adverse effects areextremely rare in horses receiving systemic multi-agentchemotherapy (unpublished observations).

Other chemotherapy protocols have included singleagent L-asparaginase i.m. once every 2–3 weeks, singleagent cyclophosphamide i.v. once every 2–3 weeks andcombinations of either cytosine arabinoside orcyclophosphamide with prednisolone (Schneider 1997).Corticosteroids, such as oral or parenteral dexamethasone,used alone have also proved helpful in reducing lymphomamasses and controlling some clinical signs temporarily (for up to 18 months) in some cases (Schneider 1997;McClure 2000; Carlson 2002; Gerber et al. 2002).Corticosteroids are also indicated to treat immune mediatedanaemia and thrombocytopenia that may occur inassociation with lymphoma.

Chemotherapy safety

Cytotoxic drugs have a very narrow margin of safety, anduse of proper personal protection equipment duringpreparation and chemotherapy administration is necessaryto minimise exposure of personnel to the drugs (Lucroy2002). Disposal of materials used should also follow theappropriate guidelines (Yodaiken and Bennett 1986). Drugssuch as 5-FU are commonly dispensed to owners for topicaltreatment over a prolonged time course. As the drug ismutagenic and carcinogenic, safety guidelines for handlingthe drug and for disposal of contaminated dressings mustbe followed by the owners.

Acknowledgements

The authors thank Dr Derek Knottenbelt for providing Figures 2and 3.

Manufacturers’ addresses

1D.C.Knottenbelt, University of Liverpool, Neston, South Wirral, UK.2Larson Labs Inc., Vet Line, Fort Collins, Colorado, USA.3NIES Inc., Las Vegas, Nevada, USA.4Camrosa Equestrian Ltd., Wadhurst, East Sussex, UK.

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