• View in gallery

    Illustration depicting the main metabolic pathways for ifosfamide (3-[2-chloroethyl]-2[(2-chloroethyl)amino]tetrahydro-2H-1,3,2-oxazaphosphorine 2-oxide). Ifosfamide differs from cyclophosphamide in that 1 of the 2 chloroethyl groups of ifosfamide is shifted to the endocyclic nitrogen.

  • View in gallery

    Graph of the plasma ifosfamide concentration-versustime curve derived from samples obtained from 9 cats after IV administration of a single dose of ifosfamide (900 mg/m2 of body surface area) infused during a 30-minute period. Time 0 was the start of the ifosfamide infusion.

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Phase I trial and pharmacokinetic analysis of ifosfamide in cats with sarcomas

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  • 1 Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.
  • | 2 Harrington Oncology Program, School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.
  • | 3 Harrington Oncology Program, School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.
  • | 4 Harrington Oncology Program, School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.
  • | 5 Section of Clinical Pharmacology, Department of Medicine, Dartmouth Medical School and Dartmouth Hitchcock Medical Center, Lebanon, NH 03756.
  • | 6 Section of Clinical Pharmacology, Department of Medicine, Dartmouth Medical School and Dartmouth Hitchcock Medical Center, Lebanon, NH 03756.
  • | 7 Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

Abstract

Objective—To determine the maximally tolerated dose (MTD) and dose-limiting toxicosis (DLT) of ifosfamide in tumor-bearing cats.

Animals—38 cats with resected, recurrent, or metastatic sarcomas.

Procedure—The starting dosage of ifosfamide was 400 mg/m2 of body surface area, IV, and dosages were increased by 50 to 100 mg/m2 in cohorts of 3 cats. To protect against urotoxicosis, mesna was administered at a dosage equal to 20% of the calculated ifosfamide dosage. Diuresis with saline (0.9% NaCl) solution before and after administration of ifosfamide was used to minimize nephrotoxicosis. Samples for pharmacokinetic analysis were obtained after the MTD was reached.

Results—38 cats were entered into this phase I study and were administered a single dose of ifosfamide at various dosages. The MTD was 1,000 mg/m2, and neutropenia was the DLT. Seven of 8 episodes of neutropenia were on day 7 after treatment, and 1 cat developed severe neutropenia on day 5. Adverse effects on the gastrointestinal tract were generally mild and self-limiting, the most common of which was nausea during ifosfamide infusion. One cat had signs consistent with a drug-induced hypersensitivity reaction. There were no episodes of hemorrhagic cystitis or nephrotoxicosis. Correlations between pharmacokinetic variables and ifosfamide-associated toxicoses were not found. Preliminary evidence of antitumor activity was observed in 6 of 27 cats with measurable tumors.

Conclusions and Clinical Relevance—The dosage of ifosfamide recommended to treat tumor-bearing cats is 900 mg/m2 every 3 weeks. This dosage should be used in phase II clinical trials.

Abstract

Objective—To determine the maximally tolerated dose (MTD) and dose-limiting toxicosis (DLT) of ifosfamide in tumor-bearing cats.

Animals—38 cats with resected, recurrent, or metastatic sarcomas.

Procedure—The starting dosage of ifosfamide was 400 mg/m2 of body surface area, IV, and dosages were increased by 50 to 100 mg/m2 in cohorts of 3 cats. To protect against urotoxicosis, mesna was administered at a dosage equal to 20% of the calculated ifosfamide dosage. Diuresis with saline (0.9% NaCl) solution before and after administration of ifosfamide was used to minimize nephrotoxicosis. Samples for pharmacokinetic analysis were obtained after the MTD was reached.

Results—38 cats were entered into this phase I study and were administered a single dose of ifosfamide at various dosages. The MTD was 1,000 mg/m2, and neutropenia was the DLT. Seven of 8 episodes of neutropenia were on day 7 after treatment, and 1 cat developed severe neutropenia on day 5. Adverse effects on the gastrointestinal tract were generally mild and self-limiting, the most common of which was nausea during ifosfamide infusion. One cat had signs consistent with a drug-induced hypersensitivity reaction. There were no episodes of hemorrhagic cystitis or nephrotoxicosis. Correlations between pharmacokinetic variables and ifosfamide-associated toxicoses were not found. Preliminary evidence of antitumor activity was observed in 6 of 27 cats with measurable tumors.

Conclusions and Clinical Relevance—The dosage of ifosfamide recommended to treat tumor-bearing cats is 900 mg/m2 every 3 weeks. This dosage should be used in phase II clinical trials.

Ifosfamide (3-[2-chloroethyl]-2[(2-chloroethyl) amino] tetrahydro-2H-1,3,2-oxazaphosphorine 2-oxide) is a member of the oxazaphosphorine family of alkylating agents and is widely used in the treatment of adults and children with various solid tumors and hematologic malignancies.1 The chemical structure of ifosfamide differs from that of another oxazaphosphorine alkylating agent, cyclophosphamide. In contrast to cyclophosphamide, in which both chloroethyl groups are attached to the same exocyclic nitrogen, 1 of the 2 chloroethyl groups of ifosfamide is shifted to the endocyclic nitrogen (Figure 1). As a result of this minor structural difference, ifosfamide is a more water-soluble compound than cyclophosphamide and possesses a different spectrum of clinical antitumor activity and toxicity.2

Figure 1—
Figure 1—

Illustration depicting the main metabolic pathways for ifosfamide (3-[2-chloroethyl]-2[(2-chloroethyl)amino]tetrahydro-2H-1,3,2-oxazaphosphorine 2-oxide). Ifosfamide differs from cyclophosphamide in that 1 of the 2 chloroethyl groups of ifosfamide is shifted to the endocyclic nitrogen.

Citation: American Journal of Veterinary Research 67, 3; 10.2460/ajvr.67.3.510

Similar to cyclophosphamide, ifosfamide is a prodrug that needs to undergo activation by hepatic CYP-3A4 to convert it to its active form, 4-hydroxyifosfamide (Figure 1). The active form (4-hydroxyifosfamide) is in balanced equilibrium with its tautomeric form, aldoifosfamide. Aldoifosfamide undergoes β elimination to yield phosphoramide mustard and acrolein. Carboxyifosfamide and 4-ketoifosfamide are 2 other inactive ifosfamide metabolites formed by metabolism via CYP-3A4. Ifosfamide is also subject to an alternative cytochrome P450–catalyzed side-chain oxidation reaction that generates therapeutically inactive dechloroethyl ifosfamide and chloroacetaldehyde. Ifosfamide and its metabolites are excreted by the kidneys.3 Phosphoramide mustard is the major cytotoxic moiety formed from ifosfamide and causes interstrand cross-links, which suggests that as a result of this DNA damage, cells not capable of repair will be eliminated by apoptosis. Analysis of in vitro data also suggests that treatment with ifosfamide leads to necrotic cell death.4

In human patients, the dose-limiting toxic effects of ifosfamide are myelosuppression and renal dysfunction.2 Neurotoxicosis manifested as cerebellar ataxia, confusion, coma, hallucinations, and seizures is probably caused by the metabolite chloroacetaldehyde and may be evident in 10% to 40% of patients receiving high doses of ifosfamide.5 Alopecia, nausea, and vomiting may also develop in patients receiving ifosfamide.6 Rare instances of hypersensitivity, pancreatitis, cardiotoxicosis, and interstitial pneumonia also have been reported.7

Severe hemorrhagic cystitis caused by the ifosfamide catabolite acrolein initially was the dose-limiting adverse effect of ifosfamide.8 Administration of the thiol compound mesna (sodium 2-mercaptoethanesulfonate) significantly reduces the incidence of hemorrhagic cystitis. In humans, mesna is transformed into an inert disulfide, dimesna, immediately after administration. Dimesna becomes a free thiol only after glomerular filtration, where it combines with acrolein in the urinary tract to form inert nontoxic condensation products.2 The use of mesna in ifosfamide-based treatment regimens has resulted in the reduction of event rates for macrohematuria and microhematuria to < 5% and 20%, respectively.9

The principal objectives of the study reported here were to determine the DLT and MTD of ifosfamide when administered as a single IV dose and to characterize the pharmacokinetic pattern of ifosfamide in cats. We also intended to obtain preliminary evidence of antitumor activity of ifosfamide in cats with sarcomas.

Materials and Methods

Animals

Client-owned cats were used in the study. Cats were considered eligible to receive ifosfamide when they had a resected, recurrent, or metastatic sarcoma that had been confirmed histologically; an expected survival of 4 weeks without treatment; not received chemotherapy, immunotherapy, or radiation therapy for at least 4 weeks; and adequate bone marrow, cardiac, renal, and hepatic function. The study was performed at the Sprecher Institute for Comparative Cancer Research at Cornell University College of Veterinary Medicine and the Harrington Oncology Program at Tufts University School of Veterinary Medicine. The study protocol was approved by an institutional animal care and use committee at each institution, and written informed consent was obtained from all clients.

Initial evaluation

Baseline evaluation included physical examination, a CBC count with differential and platelet counts, serum biochemical analysis, and urinalysis. Gross tumors were not required, but when appropriate, tumors were measured directly with calipers and tumor volume calculated as the number of cubic millimeters by use of the following equation: length × width × height × (π/6). The CBC and platelet counts were obtained at weekly intervals until day 21 after ifosfamide administration or until recovery of neutrophil and platelet counts, whichever was longer. Serum biochemical analysis and urinalysis were repeated on day 21 after ifosfamide administration. Evidence of toxic effects of ifosfamide was monitored by evaluation of the medical history obtained from owners and results of physical examination and laboratory data. Toxic effects were graded in accordance with predetermined criteria (Appendix).10,11 The DLT was defined as grade 2 renal toxicosis or any grade 4 hematologic, gastrointestinal, or urothelial toxicosis that developed during the first cycle of ifosfamide treatment.

Dose escalation and ifosfamide administration

The study was conducted as an open-label phase I, dose-escalation study. Each cat was administered a single dose of ifosfamide, IV. The starting dosage of ifosfamide was 400 mg/m2 of body surface area. Initially, dose escalation was planned for increments of 50 mg/m2. However, when no toxic effects were observed after administration of 600 mg/m2, additional dose escalation used increments of 100 mg/m2.

Each dosage was administered to 3 cats, provided that none of the treated cats had DLT. When 1 of the 3 cats in a group had DLT, 3 additional cats were also administered ifosfamide at that same dosage. When no DLT was observed in the additional 3 cats, the dosage was escalated. When 2 or more cats in a group had DLT, at least 3 additional cats were administered the preceding treatment dosage. The MTD was defined as the dosage at which ≥ 2 of 3 or ≥ 2 of 6 cats had DLT during their first cycle of treatment. The dosage recommended for phase II trials of ifosfamide in cats was defined as the highest dosage below the MTD that resulted in ≤ 1 of 6 cats with DLT.

The amount of ifosfamidea was reconstituted in saline (0.9% NaCl) solution at a volume 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 at a concentration of 20 mg/mL. The drugs were administered, IV, through an indwelling catheter inserted in a cephalic vein; saline solution was administered for diuresis in conjunction with drug administration. Specifically, cats received a bolus of mesna, IV, followed by diuresis with saline solution (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 another 5 hours after completion of ifosfamide administration, with additional doses of mesna injected 2 and 5 hours after completion of the ifosfamide administration. This procedure was designed on the basis of results that our laboratory group obtained in a study10 on the use of ifosfamide in dogs.

Pharmacokinetic analysis

After the MTD of ifosfamide in cats was reached and the dosage for evaluation during phase II studies was determined, samples for pharmacokinetic analysis were obtained from 9 cats. An indwelling catheter was inserted in the jugular vein of each cat, and blood samples (1 mL) were collected into heparinized tubes before and 20, 25, 30, 45, 60, 120, and 240 minutes after start of the ifosfamide infusion. Samples were immediately centrifuged, and plasma was harvested and frozen at −70°C until analyzed.

The assay to determine ifosfamide concentrations in feline plasma was a modification of the assay described elsewhere.12 Briefly, 100 μL of feline plasma was spiked with 10 μL of a cyclophosphamide solution (1 mg/mL) and extracted by use of columns containing cyclohexylc (100 mg). Loaded columns were initially washed with a solution (10:90) of 1 mL of acetonitrile:10mM KH2PO4 (pH, 4). Column beds were then eluted with a solution (40:60) of 500 μL of acetonitrile:25mM KH2PO4 (pH,4), and eluents were centrifuged (13,000 × g for 1 minute) to remove any particulates. The high-performance liquid chromatography system consisted of an autosampler,d dual pumps,e a 250 × 4.6-mm C8 (5-μm) column,f and a 30 × 4.6-mm RP-8 (5-μm) guard.g A UV detectorh and chromatography workstation softwarei run on a specified platformj with a commercially available programk were used to store the chromatograms. The mobile phase was isocratic and consisted of a solution (23:77) of acetonitrile:25mM KH2PO4 (pH, 4). Flow rate was 1 mL/min at 20°C. The UV detection was set at 200 nm. Eluent samples (150 μL) were introduced by use of the autosampler. Retention time for ifosfamide was 16.5 minutes and for cyclophosphamide (internal standard) was 18.4 minutes. The standard curve was linear for the range from 10 to 100 μg of ifosfamide/mL. This ifosfamide assay had a limit of detection of 10 μg/mL. Interday and intraday coefficients of variation for the range of the standard curve varied from 8.4% to 12.9% and from 1.9% to 7.5%, respectively. Accuracy of the assay ranged from 94.6% to 105.6%.

The plasma ifosfamide concentrations for each cat after a single IV administration of ifosfamide infused during a 30-minute period were inspected on a semilogarithmic plot of ifosfamide concentration versus time. Values for Cmax and the time at which Cmax was achieved were determined by visual examination of raw data for the plasma ifosfamide concentration–time plot. Pharmacokinetic variables were estimated by use of a computer software program.l The ifosfamide concentration–time data were analyzed by use of an open noncompartmental method with a constant infusion by use of a computerized program.m The value for ke was determined by use of linear regression of the terminal 3 to 6 points of the logarithmic plasma ifosfamide concentration-versus-time plot with a weighted paradigm of 1/Y2, where Y is the plasma ifosfamide concentration. The value for t1/2 was estimated as 0.693/ke. The AUC to the last data point was estimated by use of the linear-logarithmic trapezoidal rule, and AUC extrapolated to infinity was determined by adding the Wagner-Nelson correction (ie, ifosfamide concentration at the last data point/ke). The value for CL was calculated as dose/AUC extrapolated to infinity. The apparent volume of distribution was estimated as CL/ke. The MRT was estimated as AUMC/AUC. The Vdss was estimated as CL × MRT.

Statistical analysis

A 1-tailed Spearman rank correlation test was used to examine whether ifosfamide-associated neutropenia or gastrointestinal toxicoses increased in severity with the ifosfamide AUC or Cmax For all analyses, calculations were performed by use of a computer software programn and values of P ≤ 0.05 were considered significant.

Results

Animals

Between February 2000 and September 2002, 38 cats with sarcomas were entered into the phase I trial and received ifosfamide at 9 dosages. There were 21 males and 17 females. Cats ranged from 2 to 16 years of age (mean, 10 years; median, 10 years) and weighed between 3.2 and 13.3 kg (mean, 5.3 kg; median, 5.6 kg). The primary tumor was located at a site potentially used for vaccination (ie, neck or interscapular area, thigh, gluteal region, dorsal lumbar area, flank, and dorsolateral thoracic area) in 30 cats, antebrachium in 4 cats, oral cavity in 3 cats, and face in 1 cat.

Twenty-eight of 38 (74%) cats had fibrosarcomas, 3 (8%) had undifferentiated sarcomas, 2 (5%) had spindle cell sarcomas, and 1 (3%) each had a nerve sheath tumor, osteosarcoma, chondrosarcoma, myxosarcoma, and giant cell sarcoma. Three tumors originated in the oral cavity (2 fibrosarcomas and 1 osteosarcoma), whereas the remaining 35 tumors were soft tissue sarcomas. Duration of disease before treatment with ifosfamide ranged from 12 to 1,210 days (mean, 248 days; median, 120 days). Seventeen (45%) cats had no prior treatment before administration of ifosfamide, 12 (32%) had been treated by use of surgery, 4 (10%) had received radiation therapy, 3 (8%) had received chemotherapy combined with radiation therapy, and 2 (5%) had received chemotherapy alone. Twenty-seven (71%) cats had macroscopic tumors at the time of treatment with ifosfamide; tumor volume in these cats ranged from 1 to 461 cm3 (mean, 40 cm3; median, 14 cm3).

Determination of MTD and DLT

All cats were used for assessment of toxic effects evident after 1 ifosfamide treatment. Dosages of ifosfamide were escalated for each successive group of cats. Myelosuppression was the principal toxic effect (Table 1). At a dosage of 1,000 mg/m2, 2 of 5 cats had DLT, which met the MTD criterion of ≥ 2 of 6 affected cats at a dosage. Thus, we concluded that the MTD was achieved.

Table 1—

Dose-escalation for ifosfamide administered to cats with various sarcomas and resulting toxic effects.

Dosage (mg/m2 of body surface area)No. of cats treatedNo. of cats with DLT
40030
45030
50030
55030
60030
70030
80030
90030
1,00052*
90092*

The DLT was grade 4 neutropenia.

†After 2 of 5 cats had a DLT at a dosage of 1,000 mg/m2, the preceding dose (ie, 900 mg/m2) was administered to 9 additional cats to determine the pattern of toxic effects.

The dosage recommended for phase II trials of ifosfamide in cats was defined as the highest dose below the MTD that resulted in ≤ 1 of 6 cats with DLT. Once we determined that 1,000 mg/m2 was the MTD, 9 cats were then administered ifosfamide at a dosage of 900 mg/m2 to further evaluate toxic effects and define the pharmacokinetic pattern. Two of these 9 cats also had DLT in the form of neutropenia (grade 4). On the basis of these results, 900 mg/m2 was determined to be the dosage of ifosfamide for use in future phase II trials in cats.

Hematologic toxicoses

Myelosuppression was the DLT (Table 2). The toxic effects recorded represent the maximum grade of toxicosis observed for a specific cat during the first cycle of treatment with ifosfamide. Seven of 8 episodes of neutropenia (grades 2 to 4) were detected on day 7 after treatment. One cat developed a fever and grade 4 neutropenia on day 5 after treatment. This cat had received ifosfamide at a dosage of 1,000 mg/m2, but it recovered after receiving supportive treatment consisting of IV administration of fluids and antimicrobials. Six cats treated with ifosfamide had unresolved neutropenia on day 21 after treatment (4 had grade 1, and 2 had grade 2) but had neutrophil counts within the reference range when rechecked on day 28 after treatment. Five of 6 episodes of thrombocytopenia (grades 2 to 4) were detected on day 7 after treatment, and 1 episode of grade 2 thrombocytopenia was detected on day 14 after administration of ifosfamide at a dosage of 550 mg/m2. All episodes of thrombocytopenia resolved by day 21 after treatment.

Table 2—

Hematologic toxicoses in cats 7 to 28 days after IV administration of a single dose of ifosfamide.

Dosage (mg/m2)No. of catsNeutropenia*Thrombocytopenia*
0123401234
40032100030000
45031200012000
50032100012000
55030210020100
60032100030000
70033000012000
80033000030000
90032001020100
1,00050201203020
90091510252200

Each category of toxicosis was graded on a scale of 0 to 4; assigned grade represents the maximum grade recorded for a specific cat during the first cycle of treatment.

Represents results for 9 additional cats administered ifosfamide at a dosage of 900 mg/m2 to determine the pattern of toxic effects.

Nonhematologic toxicoses

Nonhematologic toxic effects were observed after treatment (Table 3). The most common nonhematologic toxic effect was signs of nausea during the ifosfamide infusion (4 cats salivated during the 30-minute infusion of ifosfamide). This clinical sign resolved after the infusion was completed. Other adverse gastrointestinal effects (eg, loss of appetite, vomiting, and diarrhea) were mild and selflimiting. None of the cats developed clinical signs of hemorrhagic cystitis, and there was no evidence of microscopic hematuria found in any of the urine sediments analyzed; however, urinalyses were only performed on day 21 after treatment. Serum biochemical analysis was repeated on day 21 after treatment in all cats, and no renal or hepatic dysfunction was observed. One cat that received ifosfamide at a dosage of 1,000 mg/m2 developed signs consistent with a hypersensitivity reaction (severe facial swelling and pruritus). These signs were evident immediately after completion of the ifosfamide infusion and resolved within 1 hour after IV administration of dexamethasone and IM administration of diphenhydramine.

Table 3—

Nonhematologic toxicoses in cats after IV administration of a single dose of ifosfamide.

Dosage (mg/m2)No. of catsAnorexia*Vomiting*Diarrhea*
012340123401234
4003300003000030000
4503300003000030000
5003300002100020000
5503300003000030000
6003300003000030000
7003300002010020100
8003300003000030000
9003300002100030000
1,0005500003110040100
900†9602107110081000

See Table 2 for key.

Pharmacokinetics

Noncompartmental pharmacokinetic variables for ifosfamide were calculated for 9 cats administered ifosfamide at a dosage of 900 mg/m2 (Table 4). The logarithmic plasma ifosfamide concentration-versus-time curve was plotted (Figure 2).

Table 4—

Noncompartmental pharmacokinetic variables for ifos-famide in 9 cats after IV administration of a single dose (900 mg/m2 infused during a 30-minute period).

VariableMean ± SDMedianRange
Cmax (μg/mL)122.8 ± 37.0100.281.8178.5
t1/2 (h)3.0 ± 1.62.51.66.7
AUC ([μg • h]/mL)479.9 ± 346.9284.3137.31,181.1
CL (L/h)0.91 ± 0.580.790.182.09
Vdss (L)4.21 ± 2.263.472.219.64
MRT (h)4.2 ± 2.33.52.29.7
Figure 2—
Figure 2—

Graph of the plasma ifosfamide concentration-versustime curve derived from samples obtained from 9 cats after IV administration of a single dose of ifosfamide (900 mg/m2 of body surface area) infused during a 30-minute period. Time 0 was the start of the ifosfamide infusion.

Citation: American Journal of Veterinary Research 67, 3; 10.2460/ajvr.67.3.510

There were no significant relationships between the pertinent pharmacokinetic variables reflecting drug exposure (Cmax and AUC) and the principal toxic effects (neutropenia and gastrointestinal toxicosis).

Antitumor activity

For inclusion in the study reported here, cats did not need to have tumors of measurable volume, and assessment of tumor response to ifosfamide treatment was not the primary goal of our study. Nonetheless, 27 cats had measurable tumors, and 6 cats had partial responses characterized by a reduction of ≥ 50% in tumor burden. Four of the 6 cats that achieved a partial response received ifosfamide at a dosage of 900 mg/m2, 1 received ifosfamide at 1,000 mg/m2, and 1 received ifosfamide at 550 mg/m2.

Of the cats that achieved a partial remission, 5 had vaccine-associated sarcomas and 1 had an undifferentiated soft tissue sarcoma located on the head.

Discussion

On the basis of the findings reported here, a dosage of 900 mg of ifosfamide/m2 administered every 3 weeks appears to be appropriate for tumor-bearing cats. This dosage of ifosfamide is considerably higher than that used to treat dogs and lower than that used in people. Dosages of 375 mg/m2 administered IV every 3 weeks as a single dose can be safely given to dogs.10 In a phase I study13 of a continuous infusion of ifosfamide for 4 days in people with cancer, dose escalation to a maximum of 18 g/m2 was possible, and the DLTs were renal dysfunction and neurotoxicosis. For ifosfamide infusions for 10 days, the maximum cumulative dose decreased to 12 g/m2, and in a phase I trial of protracted infusion of ifosfamide for 14 days,14 the MTD was 7 g/m2 per cycle. Currently, schedules for ifosfamide administration in human oncology range from 1.8 to 3 g/m2/d for 2 to 5 days for a total dose of 6 to 15 g/m2 every 3 weeks; ifosfamide is administered IV, preferably during a period of 1 to 3 hours or as a continuous infusion during a period of 24 hours.15

Interspecies differences in the tolerated ifosfamide dose may perhaps be explained by differences in infusion schedules and cytochrome P450–mediated ifosfamide metabolism. A high single dose may saturate activating enzymes, whereas a fractionated dose schedule may permit higher amounts of drug to be activated per treatment course.16,17 Also, the relative affinity of ifosfamide for activating enzymes as well as the rate of activation by cytochrome P450 may differ among species. Finally, ifosfamide can autoinduce its own metabolism when administered to humans in accordance with a fractionated schedule of daily administration for 5 days,18,19 but it is unknown whether this phenomenon is evident in all species. Although actual comparative studies of ifosfamide in various species have not been performed, in a study20 conducted to test various catalytic markers for cytochrome P450–mediated reactions in vitro, investigators were able to conclude that there were large differences between humans, dogs, and cats. Interspecies differences in ifosfamide dosing may be further explained by differences in drug or metabolite transport activity or differences in mechanisms of cellular resistance to alkylating agents, such as differing intracellular content of glutathione, aldehyde dehydrogenase activity, and ability to repair DNA lesions.

In the study reported here, administration of ifosfamide in conjunction with diuresis via saline solution and mesna for uroprotection resulted in myelotoxicosis, specifically neutropenia 5 to 7 days after ifosfamide treatment, being the DLT for cats. On the basis of a meta-analysis of toxicity experiments in rodents,21 myelotoxic effects of cyclophosphamide have been correlated with the Cmax of 4-hydroxycyclophosphamide, the active metabolite generated during hepatic metabolism of cyclophosphamide. The same may be true for ifosfamide. We conducted exploratory analyses correlating ifosfamide pharmacokinetic variables with toxic effects, and although we did not detect any relationships, these observations must be interpreted with caution because of the relatively small number of patients from which we obtained pharmacokinetic data and the fact that we measured the concentration of ifosfamide only and not its metabolites.

Glomerular and tubular dysfunctions represent serious potential adverse effects of high cumulative doses of ifosfamide, especially in children.22 We did not observe any evidence of renal toxicosis in cats in the dose-escalation study reported here, but only 1 dose of ifosfamide was administered to each cat. However, in a phase II study23 of ifosfamide in 27 cats with sarcomas, 2 developed severe nephrotoxicosis. Interestingly, ifosfamideinduced nephrotoxicosis has been correlated with the formation of the metabolite chloroacetaldehyde,24 and this is the same ifosfamide metabolite responsible for neurotoxic effects in people.5 However, none of the cats in the study reported here or a follow-up phase II study23 had any clinical evidence of CNS toxicosis.

Hemorrhagic cystitis is another potential adverse effect of ifosfamide treatment in people,22 but we have not observed any evidence of microscopic or macroscopic hematuria in treated cats. In people with cancer, ifosfamide has a greater tendency than does cyclophosphamide to cause hemorrhagic cystitis, possibly because of the higher doses administered, which results in the production of higher quantities of acrolein.22 Hemorrhagic cystitis secondary to cyclophosphamide administration in cats is rare,25,26 so the feline uroepithelium is relatively resistant to acrolein-induced damage, substantial amounts of acrolein are not produced during ifosfamide metabolism in cats, or the mesna-diuresis protocol used in the study reported here prevented this complication.

One cat had signs of a drug-induced hypersensitivity reaction. Nearly all chemotherapeutic agents have caused an isolated instance of a hypersensitivity reaction, and hypersensitivity reactions have been reported27,28 after administration of ifosfamide in humans. Until more information becomes available, cats administered ifosfamide should be closely monitored during treatment.

Ifosfamide is considered the second most active agent for use in treating humans with soft tissue sarcomas.29 Phase II trials conducted in Europe and the United States have revealed complete or partial response rates ranging from 20% to 40% in previously treated patients and 27% to 86% in untreated patients.15 As previously mentioned, ifosfamide is administered IV to humans as a bolus or as a continuous infusion for up to 5 days. In principle, continuous infusion is superior for tumors with a low growth fraction, such as soft tissue sarcomas. The rate of ifosfamide cytochrome P450 activation is relatively slow and may be saturable, and ifosfamide can autoinduce its own metabolism when administered as fractions over a period of days.16–19 Thus, long-term continuous infusion of the drug would seem to be more favorable, compared with bolus treatments. Although phase III trials to address this issue have not been conducted, 1 study30 in which investigators analyzed a sequential group of people with unresectable sarcomas receiving ifosfamide as a bolus and subsequent treatments as an infusion revealed that there was a higher response rate in the group receiving ifosfamide as a bolus. In the study reported here, antineoplastic activity was observed in 6 of 27 cats with measurable sarcomas treated by administration of ifosfamide at a dosage of 550 to 1,000 mg/m2.

Ifosfamide in conjunction with administration of saline solution as a diuresis infusion and mesna for urothelial protection appears to be a safe drug for use in tumor-bearing cats. Additional studies are warranted to examine the influence of the ifosfamide schedule of administration and additional information about the response of various malignancies. When such information becomes available, the role of ifosfamide-containing combinations in initial chemotherapeutic treatments should be determined.

CYP-3A4

Cytochrome P450-3A4

DLT

Dose-limiting toxicoses

MTD

Maximally tolerated dose

Cmax

Maximum plasma ifosfamide concentration

t1/2

Terminal elimination half-life

Ke

Terminal elimination rate

AUC

Area under the plasma ifosfamide concentration-versus-time curve

CL

Total body clearance

MRT

Mean residence time

AUMC

Area under the moment curve

VDss

Volume of distribution at steady state

a

Ifex, Bristol-Myers Squibb Co, Princeton, NJ.

b

Mesnex, Bristol-Myers Squibb Co, Princeton, NJ.

c

Varian SPE, CH, Varian Inc, Palo Alto, Calif.

d

Varian Model 410, Varian Inc, Palo Alto, Calif.

e

Prostar 210, Varian Inc, Palo Alto, Calif.

f

Phenomenex Prodigy C8 (5 μm) column, Phenomenex, Torrance, Calif.

g

Perkin Elmer Brownlee Spheri-5 column, Perkin Elmer, Norwalk, Conn.

h

Varian Model 315 UV/VIS detector, Varian Inc, Palo Alto, Calif.

i

Varian Star 5.51 Chromatography workstation software, Varian Inc, Palo Alto, Calif.

j

Dell Optiplex GX 110 platform, Dell Inc, Round Rock, Tex.

k

Microsoft Windows 98SE, Microsoft Corp, Redmond, Wash.

l

WinNonlin Pro V4.1 pharmacokinetic program, Pharsight Corp, Mountain View, Calif.

m

WinNonlin model 202, Pharsight Corp, Mountain View, Calif.

n

SPSS statistical analytical software, version 10, SPSS Inc, Chicago, Ill.

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Appendix1

Criteria used to grade10,11 toxic effects in cats administered ifosfamide (3-[2-chloroethyl]-2[(2-chloroethyl)amino]tetrahydro-2H-1,3,2-oxazaphosphorine 2-oxide).

Toxic effectGradeSigns
Neutropenia0None
11,500 to 2,500 neutrophils/μL
21,000 to 1,499 neutrophils/μL
3500 to 999 neutrophils/μL
4< 500 neutrophils/μL
Thrombocytopenia0None
1100,000 to 200,000 platelets/μL
250,000 to 99,000 platelets/μL
315,000 to 49,000 platelets/μL
4< 15,000 platelets/μL
Urothelial0None
1Microscopic hematuria
2Gross hematuria, no dysuria
3Hematuria and dysuria, responds to anti-inflammatories
4Hematuria and dysuria requiring intravesical therapy
Renal0None
1Serum creatinine concentration of 2 to 3 mg/dL
2Serum creatinine concentration of 3 to 4 mg/dL
3Serum creatinine concentration of 4 to 5 mg/dL
4Serum creatinine concentration of > 5 mg/dL
Anorexia0None
1Inappetence
2Anorectic for < 3 d
3Anorectic for > 3 d but < 5 d
4Anorectic for > 5 d and weight loss of 10%
Vomiting0None
1Signs of nausea
2Sporadic and self-limiting
31 to 5 episodes/d for < 2 d
46 to 10 episodes/d; requires hospitalization
Diarrhea0None
1Soft feces and responds to dietary modification
21 to 4 episodes of watery feces/d for < 2 d
34 to 7 episodes of watery feces/d for > 2 d
4> 7 episodes of watery feces/d or bloody feces; requires hospitalization

Contributor Notes

Dr. Moore’s present address is Veterinary Oncology Consultants, 379 Lake Innes Dr, Wauchope, New South Wales 2446, Australia.

Dr. Northrup’s present address is Department of Small Animal Medicine and Surgery, College of Veterinary Medicine, University of Georgia, Athens, GA 30602.

Supported by a Health Study Grant from the Winn Feline Foundation affiliated with The Cat Fancier’s Association Incorporated.

Presented in part at the 22nd Annual Conference of the Veterinary Cancer Society, New York, September 2002.

Dr. Rassnick.