Vaccine-associated sarcomas may develop in cats at a rate of 0.63 to 3 tumor-bearing cats/10,000 cats vaccinated annually.1,2 Evidence consistently points to an increased risk among cats receiving adjuvant-containing vaccines against rabies and FeLV,1–7 but there has not been an association with a specific vaccine brand or manufacturer.3 Research efforts are underway to determine the mechanism of oncogenesis and roles of inflammatory and immune responses after vaccination.8
A VAS is locally invasive and commonly recurs after surgical excision. Up to one fourth of tumors may metastasize to the lungs and other sites.4,6,9–13 A combination of surgery and radiation therapy has increased the control rate of VASs; however, clinicians still fail to cure cats.11–13 In 1 study,13 median time to treatment failure was 584 days when cats were treated with radiation therapy before surgical resection. Treatment failure may result from local recurrence, metastasis, or both. Identifying effective chemotherapeutic agents is essential for the management of some cats with VAS. Combined treatment modalities (radiation therapy, surgery, and chemotherapy) may help to improve tumor control and patient survival. Chemotherapy may be the 1 remaining option when surgery or radiation therapy is not possible or has not been successful. However, there have been few controlled studies14,15 to evaluate the response of VASs to various chemotherapeutic agents.
Ifosfamide (3-[2-chloroethyl]-2[(2-chloroethyl) amino]tetrahydro-2H-1,3,2-oxazaphosphorine 2-oxide) is an oxazaphosphorine nitrogen mustard developed as an isomer of the alkylating agent cyclophosphamide. As a result of minor structural differences, the spectrum of clinical antitumor activity and toxic effects differs between ifosfamide and cyclophosphamide.16 Ifosfamide is currently included in the standard treatment of children with bone and soft tissue sarcomas and is used in selected high-risk groups of people with Wilms' tumors, neuroblastomas, and germ cell tumors.17 Efficacy has also been reported18 against lymphomas, transitional cell carcinoma of the bladder, tumors of the breasts and lungs, gynecologic cancers (ovarian or cervical), carcinomas of the head and neck, and thymic tumors.
In another study19 conducted by our laboratory group, we performed a phase I trial to determine the maximally tolerated dose, dose-limiting toxicoses, and the pharmacokinetic pattern of ifosfamide in cats with various sarcomas. The objective of the phase II trial reported here was to evaluate the efficacy of ifosfamide administered IV as a single dose in conjunction with infusion of saline (0.9% NaCl) solution for diuresis and mesna for urothelial protection every 3 weeks to cats with a measurable VAS.
Materials and Methods
Animals
Client-owned cats were eligible for inclusion in the study when they had a locally advanced, recurrent, or metastatic VAS that had been confirmed histologically. The diagnosis of vaccine association was made on the basis of tumor location, history of vaccination at the site of the tumor, and peritumoral lymphocytes and macrophages detected during histologic examination.6 Cats with prior surgical excision of a tumor were allowed to enter the study, whereas cats that had received chemotherapy, radiation therapy, immunotherapy, or a combination of these treatments were excluded from the study. Additional inclusion criteria included adequate hematologic, cardiac, renal, and hepatic function and a life expectancy of at least 6 weeks without treatment.
The study was a prospective, multiple-institutional trial based out of Cornell University to evaluate ifosfamide in cats with VAS. Written informed consent was obtained from all clients before their cats entered the study. The study protocol was approved by the Institutional Animal Care and Use Committee at Cornell University.
Treatment protocol
All cats were initially treated by IV administration of ifosfamidea at a dosage of 900 mg/m2 of body surface area. This dosage was determined in another study19 conducted by our laboratory group to define the maximally tolerated dose of ifosfamide in cats. For any cats with a grade 4 hematologic (neutrophils < 500 cells/μL or platelets < 15,000 platelets/μL), gastrointestinal (anorectic for > 5 days; vomiting or diarrhea requiring hospitalization), or urothelial (hematuria or dysuria) toxicosis,19 subsequent ifosfamide dosages were reduced by 25%. Ifosfamide was discontinued when a cat developed grade 2 renal toxicosis (serum creatinine concentration ≥ 3 mg/dL). The prescribed dose of ifosfamide was reconstituted in saline solution to achieve a volume for infusion of 9.15 mL/kg. Mesnab for urothelial protection was administered at a dosage equal to 20% of the calculated ifosfamide dosage and was reconstituted in saline solution to achieve a concentration of 20 mg/mL. Drugs were administered IV through an indwelling catheter in conjunction with diuresis with saline solution. Specifically, cats received a bolus of mesna, IV, followed by diuresis with saline solution at a rate of 18.3 mL/kg/h, IV, for 30 minutes. The calculated dose of ifosfamide was then infused during a 30-minute period. Diuresis with saline solution was continued for an additional 5 hours, with repeated doses of mesna administered 2 and 5 hours after completing the ifosfamide infusion. Treatments were administered every 21 days, provided cats had sufficiently recovered from any drug-related toxic effects associated with the preceding treatment. It was planned that each cat would receive at least 2 treatments. When no response to treatment was observed after 2 consecutive ifosfamide infusions, clients were given the opportunity to pursue other treatment options for their cats. Cats that achieved a measurable response to treatment after 2 ifosfamide infusions were administered up to 2 additional ifosfamide infusions every 21 days.
Evaluation of tumor response
Before treatment, tumors were measured with calipers, and tumor volume was recorded in cubic millimeters by use of the following equation: length × width × height × (π/6). Cats were examined weekly after treatment, and tumor response was determined at each examination by measuring the tumors. Response to treatment was categorized as a complete response (100% reduction in size of all measurable tumors), partial response (≥ 50% reduction but < 100% reduction in size of all measurable tumors), stable disease (< 50% reduction or no change in size of all measurable tumors and lack of appearance of new neoplastic lesions), or progressive disease (increase of ≥ 25% in the size of all measurable tumors or the appearance of new neoplastic lesions). We required that all responses last for at least 21 days; decreases in tumor size that lasted for < 21 days were defined as stable disease.
Evaluation of ifosfamide-associated toxicoses
All cats underwent pretreatment evaluation that included a baseline CBC count with differential and platelet counts, serum biochemical analysis, and urinalysis. The CBC and platelet counts were conducted on days 7, 14, and 21 after each treatment or until neutrophil and platelet counts recovered, whichever was longer. Additionally, CBC and platelet counts were conducted on day 5 after the first treatment. Serum biochemical analysis and urinalysis were repeated on day 21 after each treatment. Evidence of ifosfamide-associated toxic effects was monitored by evaluation of laboratory data, results of physical examination, and medical history obtained from the owners. All toxicoses were graded in accordance with criteria described elsewhere.19
Statistical analysis
All eligible cats that began treatment with ifosfamide were considered for use in estimating the tumor response rate and duration. Overall response rate was defined as the number of cats achieving complete or partial response, compared with the number of cats treated. Complete and partial response rates were defined as the number of cats achieving a complete response or partial response, respectively, compared with the number of cats treated. On the basis of Simon's optimal 2-stage design for phase II trials,20 the target sample size for the study reported here was 24 cats with VAS. Our goal was to detect a lower overall response rate of 0.05 and target overall response rate of 0.25 by use of an α (probability of accepting a drug with response probability for the lower overall response rate) of 0.10 and β (probability of rejecting a drug with response probability for the target overall response rate) of 0.10. A minimum of 1 cat with a confirmed response in the initial 9 cats treated (ie, stage 1) was considered sufficient to proceed to the second stage of accrual. A minimum of 3 cats with confirmed responses in a total of 24 treated cats (ie, stages 1 and 2) was considered sufficient evidence of promising activity of the drug in this patient population.20
For cats achieving a complete or partial response, response duration was calculated as the number of days from the time of maximal response until progression of disease by use of the Kaplan-Meier method with censoring of patients as a result of termination of follow-up monitoring. The 95% CIs were determined for response proportion21 and response duration.22
Hematologic toxic effects were summarized via summary statistics, and hematologic nadirs were reported as a minimum value for each cat and each treatment. Nonhematologic toxic effects were summarized as a maximum grade for a specific type of event for each treatment.
Responders (complete or partial) were compared with nonresponders (stable or progressive disease) with respect to age, body weight, tumor volume, duration of disease before administration of ifosfamide, prior treatment (none vs surgery), sex (male vs female), ifosfamide-associated neutropenia (yes vs no), and ifosfamide-associated gastrointestinal toxicosis (yes vs no). Student t tests were used for analyses of continuous data that were normally distributed. The Mann-Whitney U test was used to analyze continuous data that were not normally distributed, and χ2 tests were used for analysis of categoric data.23 For all analyses, values of P ≤ 0.05 were considered significant. All statistical calculations were performed by use of a computer software program.c
Results
Animals
Twenty-seven cats with VAS from 6 institutions were enrolled in this phase II trial between October 2001 and January 2003. Cats (15 males and 12 females) ranged from 6 to 18 years of age (mean, 11 years; median, 11 years) and weighed between 3.3 and 11.1 kg (mean, 5.0 kg; median, 5.6 kg). Tumors were located in the neck and interscapular area (n = 22), gluteal and thigh area (2), dorsal lumbar region (2), and flank (1). Histologic evaluation revealed 21 fibrosarcomas, 2 undifferentiated sarcomas, 2 osteosarcomas, 1 chondrosarcoma, and 1 myxofibrosarcoma.
Duration of disease ranged from 1 to 777 days (mean, 67 days; median, 146 days). Fifteen (56%) cats were treated by use of surgery before treatment with ifosfamide. Mean and median durations of time from recurrence of disease and treatment with ifosfamide in these cats were 244 and 184 days, respectively (range, 16 to 777 days). The remaining 12 (44%) cats had not received treatment before administration of ifosfamide. Mean and median durations of disease before treatment with ifosfamide in these 12 cats were 23 and 29 days, respectively (range, 1 to 44 days).
All cats had measurable tumors at the time of ifosfamide treatment. Tumor volume was not calculated for 1 cat with radiographic evidence of diffuse pulmonary metastases. For the remaining 26 cats, tumor volume ranged from 0.9 to 1,029 cm3 (mean, 111 cm3; median, 34 cm3).
Treatments and toxicoses
Sixty-one ifosfamide treatments were administered to the 27 cats (range, 1 to 4; mean, 2; median, 2). In 2 cats, only 1 treatment was administered because of rapidly progressive disease. For the remaining cats, 18 received 2 treatments, 5 received 3 treatments, and 2 received 4 treatments.
Dose reductions of ifosfamide were used in 6 of 61 (10%) treatments involving 5 (19%) cats (4 cats because of neutropenia and 1 cat with anorexia). One cat had a neutrophil nadir of 390 cells/μL on day 7 after the first treatment, and despite a dose reduction in ifosfamide of 25%, that cat had a neutrophil count of < 100 cells/μL on day 7 after the second treatment. Despite another dose reduction of 25%, the cat still had severe neutropenia (198 cells/μL) after the third treatment. Because of progressive disease, no further treatments were administered to that cat. Another cat had a neutrophil nadir of 290 cells/μL on day 5 after the first treatment, and despite a dose reduction of 25%, it still had a neutrophil count of 473 cells/μL after the second treatment. No further treatments were administered to that cat. Dose reductions successfully avoided severe neutropenia and anorexia when used in the other 3 cats.
Data on toxic effects for all 27 cats were summarized (Table 1). The toxicoses recorded represented the maximum grade of toxicosis observed for a specific cat after each treatment with ifosfamide. After the first treatment, the time at which the neutrophil nadir was detected varied from 5 to 28 days (mean, 10 days; median, 7 days). Neutrophil nadir was detected on day 5 (n = 5 cats), day 7 (12), day 14 (1), day 21 (4), and day 28 (1) after the first ifosfamide treatment. Time of the neutrophil nadir was not available for 4 cats because of incomplete data attributable to poor owner compliance.
Treatment-related adverse effects after ifosfamide administration* to 27 cats with VAS.
| Toxic effect† | Treatment 1 (n = 27) | Treatment 2 (25) | Treatment 3 (7) | Treatment 4 (2) | ||||
|---|---|---|---|---|---|---|---|---|
| Grade 1 or 2 | Grade 3 or 4 | Grade 1 or 2 | Grade 3 or 4 | Grade 1 or 2 | Grade 3 or 4 | Grade 1 or 2 | Grade 3 or 4 | |
| Neutropenia | 13 | 5 | 7 | 6 | 1 | 1 | 1 | 0 |
| Thrombocytopenia | 10 | 1 | 12 | 0 | 3 | 0 | 2 | 0 |
| Renal | 0 | 0 | 0 | 2 | 0 | 0 | 0 | 0 |
| Urothelial | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| Anorexia | 3 | 3 | 3 | 1 | 0 | 1 | 0 | 0 |
| Vomiting | 3 | 0 | 3 | 0 | 1 | 1 | 0 | 0 |
| Diarrhea | 1 | 0 | 1 | 0 | 0 | 0 | 0 | 0 |
Cats received 1 to 4 treatments with ifosfamide; 61 ifosfamide treatments were administered to the 27 cats.
Results are expressed as the maximum grade of toxic effect observed for each cat for each treatment administered. Each toxic effect was graded on a scale of 0 (no effect) to 4 (maximum toxic effect), and each cat could have > 1 toxic effect after each treatment.
Nadir neutrophil counts for all 23 cats with complete hematologic data ranged from 200 to 5,382 cells/μL (mean, 2,068 cells/μL; median, 1,600 cells/μL). Neutropenia lasted 2 to 21 days after the nadir (mean, 8 days; median, 7 days). The cat with a neutrophil nadir on day 14 after the first ifosfamide treatment had a count of 2,460 cells/μL, and 3 of 4 cats with a neutrophil nadir on day 21 after the first ifosfamide treatment had counts > 2,500 cells/μL. The remaining cat with a neutrophil nadir on day 21 (1,170 cells/μL) required 14 days to return to the reference range, and the cat with a neutrophil nadir on day 28 after the first ifosfamide treatment (1,404 cells/μL) required 7 days to return to the reference range. No cats developed fever or had clinical signs consistent with sepsis after treatment.
Nadir of the platelet count was detected 5 to 21 days (mean, 8 days; median, 7 days) after the first ifosfamide treatment. Nadir platelet counts ranged from 40,000 to 366,000 platelets/μL (mean, 200,900 platelets/μL; median, 209,000 platelets/μL).
Ifosfamide-related nonhematologic toxicoses observed during treatment were summarized (Table 1). Adverse gastrointestinal effects were detected in 9 of 27 (33%) cats administered 1 to 4 doses of ifosfamide. Signs of nausea (eg, salivation) during the ifosfamide infusion and inappetence after ifosfamide treatment were the most common adverse gastrointestinal effects. After the first treatment, 7 of 27 (26%) cats had adverse gastrointestinal effects, and clinical signs were anorexia in 3 cats (grade 2, grade 3, and grade 4, respectively), evidence of nausea during the ifosfamide infusion and grade 4 anorexia in 1 cat, grade 2 anorexia and grade 2 vomiting in 1 cat, grade 2 anorexia and grade 1 diarrhea in 1 cat, and evidence of nausea during the ifosfamide infusion in 1 cat. After the second treatment, 6 of 25 (25%) cats had adverse gastrointestinal effects consisting of anorexia in 4 cats (grade 1, grade 2, grade 2, and grade 4, respectively), grade 2 vomiting in 1 cat, and evidence of nausea during the ifosfamide infusion in 1 cat. After the third treatment, 2 of 7 (29%) cats had adverse gastrointestinal effects consisting of evidence of nausea during the ifosfamide infusion in 1 cat and grade 3 vomiting and grade 3 anorexia in the other cat.
Two (7%) cats were euthanatized because of nephrotoxicosis; each of these cats had received 2 ifosfamide treatments. In retrospect, 1 cat may have had stable chronic kidney disease prior to entering the study. That cat was 16 years old and had a pretreatment serum creatinine concentration of 2.1 mg/dL (reference range, 0.5 to 1.4 mg/dL), BUN concentration of 44 mg/dL (reference range, 10 to 40 mg/dL), and urine specific gravity of 1.011. Renal variables remained stable when evaluated 21 days after the first ifosfamide treatment, but the cat was euthanatized approximately 1 month after the second treatment because of clinical laboratory results consistent with acute renal failure (serum creatinine concentration, 7.2 mg/dL; BUN concentration, 92 mg/dL; and urine specific gravity, 1.009). Histologic examination of renal tissues in that cat revealed severe, chronic interstitial nephritis and fibrosis with glomerular sclerosis. The other cat that developed renal toxicosis and was euthanatized had pretreatment renal variables within their respective reference ranges but developed clinical laboratory evidence consistent with acute renal failure (serum creatinine concentration, 10.9 mg/dL; BUN concentration, 240 mg/dL; and urine specific gravity, 1.010) approximately 3 months after entering the study. Microbial culture of urine samples yielded negative results; unfortunately, necropsy with histologic examination was not performed on this cat.
One cat developed clinical signs consistent with fluid overload and pulmonary edema (dyspnea and hypoxemia) during the diuresis infusion after ifosfamide administration of the second ifosfamide treatment. The cat was treated by administration of furosemide and supplemental oxygen and recovered as expected. Results of an echocardiograph were typical, and the cat safely received 2 more ifosfamide treatments; however, saline solution during the diuresis phase was administered at 75% of the initial rate (ie, 13.7 mL/kg/h).
None of the cats developed clinical signs of hemorrhagic cystitis. Urinalysis performed 21 days after each treatment did not yield evidence of microscopic hematuria in any of the urine sediments analyzed. Similarly, there was no evidence of glucosuria in any of the urine samples evaluated.
Response to treatment
All 27 cats had measurable tumors that were used to assess antitumor response. Overall response rate was 41% (11 of 27; 95% CI, 25% to 59%; Figure 1). One cat (16-year-old domestic shorthair cat with a myxofibrosarcoma in the interscapular area) had a complete response after 2 ifosfamide treatments. That cat was in complete remission when it was euthanatized because of renal toxicosis 57 days after treatment. Ten of 27 (37%) cats had a partial response to ifosfamide administration with a duration of 21 to 133 days (mean duration, 84 days; median duration, 70 days; 95% CI, 57 to 112 days). Two of these 10 cats were censored at 46 and 86 days, respectively, because of complications from preexisting diabetes in 1 cat and renal failure in the other cat. Treatment with ifosfamide resulted in stable disease in 11 of 27 (41%) cats. Duration of stable disease was not calculated because when an antitumor response was not observed after 2 treatments, participation in the trial was terminated. The remaining 5 (18%) cats had progressive disease.
Kaplan-Meier curve depicting duration of a complete or partial response for 11 of 27 cats with VASs treated by administration of ifosfamide. Median and mean response durations were 70 and 86 days, respectively (95% CI, 60 to 113 days).
Citation: American Journal of Veterinary Research 67, 3; 10.2460/ajvr.67.3.517
Kaplan-Meier curve depicting duration of a complete or partial response for 11 of 27 cats with VASs treated by administration of ifosfamide. Median and mean response durations were 70 and 86 days, respectively (95% CI, 60 to 113 days).
Citation: American Journal of Veterinary Research 67, 3; 10.2460/ajvr.67.3.517
Kaplan-Meier curve depicting duration of a complete or partial response for 11 of 27 cats with VASs treated by administration of ifosfamide. Median and mean response durations were 70 and 86 days, respectively (95% CI, 60 to 113 days).
Citation: American Journal of Veterinary Research 67, 3; 10.2460/ajvr.67.3.517
Of the 12 cats that had received other treatments prior to treatment with ifosfamide, 5 had a partial response and the remaining 7 had stable or progressive disease. Of the 15 cats that were treated by use of surgery prior to treatment with ifosfamide, 1 had a complete response, 5 had a partial response, and the remaining 9 had stable or progressive disease. There was no significant difference between prior treatment and response. Similarly, there were no significant differences between responders and nonresponders with respect to age, body weight, tumor volume, duration of disease, sex, or whether a cat did or did not have hematologic or gastrointestinal toxic effects.
After completion of the study, 14 cats received no additional treatment, 5 cats underwent surgical resection of the tumor, and alternative chemotherapeutic agents were administered to the remaining 6 cats. Of the 6 cats that received alternative chemotherapeutics, 3 were administered doxorubicin; 1 was administered a combination of doxorubicin, carboplatin, and mitoxantrone; 1 was administered a combination of doxorubicin and carboplatin; and 1 was administered a combination of carboplatin, mitoxantrone, and gemcitabine. One of the 6 cats that failed to respond to ifosfamide (stable disease) had a partial response for 70 days after administration of doxorubicin, whereas the remaining 5 cats had stable or progressive disease.
Discussion
A relative paucity of information exists regarding the efficacy of systemically administered chemotherapeutics to treat cats with VAS. On the basis of the results of the prospective phase II study reported here, ifosfamide caused a measurable response in 11 of 27 (41%) cats with VAS for a median duration of 70 days. Many of the cats had large tumor burdens, and more than half had tumors that were recurrent after attempted surgical excision. Although there was no significant association between response and tumor size or prior treatment, these factors can affect the evaluation of a drug. Analysis of our results suggests that ifosfamide is useful for cats with macroscopic tumors and, in addition, may be useful as an adjunctive treatment for microscopic localized tumors, systemic metastases, or both.
Adequate surgical resection with or without adjuvant external beam irradiation remains the primary treatment for cats with VAS. In 1 study,10 61 cats with VAS were treated by use of marginal (local excision with margins < 3 cm in width), wide (tumors resected with margins ≥ 3 cm in width), or radical (limb amputation) excisions, and overall median time to first recurrence of the tumor was 94 days. Median time to first recurrence was significantly longer after radical first excision (325 days) than after marginal or wide first excision (79 days). Thirteen (22%) cats developed metastases to the lungs, lymph nodes, and skin, and the overall median survival time for all cats in that study was 576 days. Most cats in that study10 were subjected to multiple surgeries or other treatments subsequent to recurrence of the tumor.
The combination of surgery and radiation therapy has improved the control of VAS in cats. One study13 of 92 cats treated with radiation therapy before surgery revealed a median time to first event (eg, local regrowth, metastases, or death) of 584 days. Median time to first event was significantly longer when no tumor cells were identified at the margin of the resected specimen (986 days) than when tumor cells were found at the margin of the resected specimen (292 days). Metastases were confirmed in 20 of 92 (21.7%) cats, and interestingly, despite the longer time to first event in cats having no tumor cells in the margins, local recurrence still developed in 42% of those cats.13 Survival as an end point was not examined separately in that study13; however, in another study,11 median survival time for 7 of 25 cats treated by surgery followed by radiotherapy was 842 days.
Doxorubicin has been considered to be the most active single agent used to treat cats with VAS. In 1 study,24 investigators reported substantial in vitro dosedependent effects of doxorubicin on the viability of 2 feline VAS cell lines. However, phase II clinical evaluations of single-agent doxorubicin in cats are limited. In 1 randomized study,15 doxorubicin and stealth liposome-encapsulated doxorubicin were evaluated in cats with VAS; there was an overall response rate of 39% and a median duration of 84 days. Five of 15 cats responded to single-agent doxorubicin (3 complete responses and 2 partial responses) and 8 of 18 responded to liposome-encapsulated doxorubicin (2 complete responses and 6 partial responses) in that study.15 The combination of doxorubicin and cyclophosphamide has also been examined in cats with nonresectable VAS. In 1 study,14 the overall response rate was 50% with a median duration of 125 days, but the sample size of 12 cats was small.
Carboplatin, a second-generation platinum compound, may prove useful in the treatment of cats with VAS, but controlled studies have not been conducted. In 1 study,14 2 cats that failed to respond to treatment with doxorubicin and cyclophosphamide subsequently were treated with carboplatin, but no objective responses were detected. In a study25 conducted to characterize the pharmacokinetic disposition and myelotoxic effects of carboplatin, 2 cats with VAS had > 50% reduction in tumor burden.
Clinical benefits of adjuvant chemotherapy in the management of cats with resected VAS have not been clearly defined.11–13,15,26 In a multiple-center study,15 75 cats with resected VAS were treated by use of adjuvant doxorubicin or liposome-encapsulated doxorubicin. When compared with results for a historical control population treated by use of surgery alone, cats receiving chemotherapy had a prolonged median disease-free survival time (388 days vs 93 days). In another study,11 18 cats with VAS were treated by use of surgery and radiation therapy followed by doxorubicin, and 7 cats were treated by use of surgery and radiation therapy; median survival time for the cats receiving chemotherapy was 674 days, compared with 842 days for the cats that did not receive chemotherapy. This value did not differ significantly between the groups; however, the power of the study (5%) was low, and more cats would be needed in each group to detect a difference, should one exist. In a study12 of 76 cats with VAS, 26 received adjuvant chemotherapy consisting of doxorubicin and cyclophosphamide. Similar to the results for an aforementioned study,11 no advantage was detected for the cats receiving chemotherapy. Recurrence rate (41%), rate of metastasis (12%), and survival interval (730 days) for cats that received chemotherapy were not significantly different from values for cats that did not receive chemotherapy.12 Similar to the results for the aforementioned study,11 analysis in this study12 was quite limited because although 26 cats received chemotherapy, many received treatment after a recurrence or development of metastases, and power to detect a difference and sample size were both extremely small. Finally, 19 of 92 cats with VAS receiving carboplatin and preoperative radiation therapy had the longest median time to first event (> 986 days vs 365 to 584 days),13 but these median values were not significantly different and were similar to values in other reports, and the nonrandomized nature of the study did not allow preselection bias to be assessed.
The dosage of ifosfamide and protocol for administration were determined on the basis of results for a phase I study19 conducted by our laboratory group. Similar to results of that phase I study19 and the use of ifosfamide in humans27 and dogs,28 results of the study reported here revealed that neutropenia was the major toxic effect. Neutropenia typically was detected 5 to 7 days after ifosfamide administration, and the median neutrophil count at the nadir was 1,600 cells/μL. Nine of 27 (33%) cats administered 1 to 4 treatments had neutrophil counts < 1,000 cells/μL; however, neutropenia was transient, and none of the cats developed signs of infection. On the basis of these results, we conclude that the recommended dosage of ifosfamide in cats is 900 mg/m2, IV, every 3 weeks. Neutropenia was documented 21 days after the first dose of ifosfamide in 2 cats, which confirmed that CBC counts should be evaluated immediately before treatment in cats receiving repeated treatments.
Two (7%) cats were euthanatized because of severe renal toxicosis. To our knowledge, renal toxicosis has not been observed in the clinical evaluation of dogs treated with ifosfamide,28,29 but moderate-tosevere ifosfamide-induced nephrotoxicosis is evident in up to 30% of treated humans.27 Nephrotoxicosis is seen most commonly in children, and several risk factors have been identified, including prior or concomitant administration of cisplatin, unilateral nephrectomy, high cumulative doses of ifosfamide, and gene mutations.30–32 Ifosfamide can induce toxic effects in any segment of a nephron, but damage to the proximal tubule is most common, which leads to increased urinary excretion of glucose, phosphorous, bicarbonate, amino acids, and protein (ie, Fanconi-like syndrome).32 The ifosfamide molecule is not nephrotoxic in vitro at clinically achievable concentrations, which suggests that ifosfamide metabolites are toxic to the kidneys. Ifosfamide undergoes 4-hydroxylation by cytochrome P450 to yield active metabolites that lead to release of acrolein, the inactive metabolite that causes urothelial toxicosis (eg, hemorrhagic cystitis). In contrast, chloroacetaldehyde is released during N-chloroethylation, which is also catalyzed by cytochrome P450. Ifosfamide-induced neurotoxic effects may develop in 10% of treated humans and has been correlated with concentrations of chloroacetaldehyde in the CNS.33 Human renal tubule cells possess cytochrome P450-3A4, the cytochrome enzyme needed to produce chloroacetaldehyde, and the amount of chloroacetaldehyde produced in renal tubule cells correlates with experimentally induced amounts associated with nephrotoxicosis.34 It has been proposed35 that chloroacetaldehyde may cause renal toxicosis by depleting intracellular concentrations of carnitine in cells of the renal tubules. Feline kidney cells have been transfected with a vector that overexpresses cytochrome P450-2B1.36 Transfected cells acquire biochemical activity and are able to activate prodrug ifosfamide, but the potential deleterious effects of inactive metabolites have not been examined. Mesna (sodium 2-mercaptoethanesulfonate) combines with acrolein in the urinary bladder and is effective in preventing urothelial toxicosis in humans treated with ifosfamide; however, it does not protect against ifosfamideinduced nephrotoxicosis.37
Other than avoiding high cumulative doses of ifosfamide, no renal protective measures exist for humans receiving ifosfamide treatment.32 Cats in the study reported here received ifosfamide along with a saline solution for diuresis, and this appeared to minimize the risk of nephrotoxic effects. Urinalysis was performed after each ifosfamide treatment, and none of the cats had evidence of glucosuria, which suggested that tubular damage was unlikely. However, cats were administered only 1 to 4 treatments, so additional studies with larger numbers of animals, a greater number of treatments with ifosfamide, and longer periods of follow-up monitoring will be needed to better define the risk of renal damage. One of the 2 cats that developed renal toxicosis in our study had evidence of chronic renal disease, which emphasizes the fact that cats with preexisting renal dysfunction should not be administered ifosfamide.
Adverse gastrointestinal effects were reported for 9 of 27 (33%) cats treated with ifosfamide. Despite this incidence of gastrointestinal toxicosis, most episodes were salivation during the infusion or transient loss of appetite after treatment. One cat developed signs of fluid overload during the diuresis phase after infusion of ifosfamide. Results of echocardiography were typical for this cat, but it is still possible that there was cardiac dysfunction. In the combined experiences of our laboratory group during a phase I study19 and the phase II study reported here, we have administered ifosfamide along with the saline solution for diuresis 99 times to 65 cats, and this was the only episode of pulmonary edema. In the phase I study19 conducted by our laboratory group, 1 cat had signs of a hypersensitivity reaction during treatment with ifosfamide, but this was not observed in any cats in the phase II study.
Surgery and radiation therapy still are the most important treatments for most cats with VAS. However, many VASs cannot be controlled by these local modalities alone. Ifosfamide should be considered for use in the treatment of cats with nonresectable, recurrent, or metastatic VAS. Additional studies should be conducted to evaluate the effectiveness of ifosfamide as an adjuvant therapy in cats treated by use of surgery with or without radiotherapy. Because of the potential for ifosfamide-induced nephrotoxicosis and the use of fluids for diuresis during administration of ifosfamide, appropriate selection of patients is warranted.
| VAS | Vaccine-associated sarcoma |
| CI | Confidence interval |
Ifex, Bristol-Myers Squibb Co, Princeton, NJ.
Mesnex, Bristol-Myers Squibb Co, Princeton, NJ.
SPSS statistical analytical software, version 10, SPSS Inc, Chicago, Ill.
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