Minimal change glomerulopathy in a cat

Abstract

A 6-year-old domestic shorthair male castrated cat was evaluated for sudden onset of vomiting and anorexia. A diagnosis of hypereosinophilic syndrome (HES) was made, and the cat was treated with imatinib mesylate. The cat had an initial clinical improvement with the normalization of the peripheral eosinophil count. After approximately 8 weeks of treatment, lethargy and anorexia recurred despite the normal eosinophil count and a significant proteinuric nephropathy was identified. Treatment with imatinib was discontinued. Ultrasound guided renal biopsies exhibited histologic, ultrastructural, and immunostaining changes indicative of a minimal change glomerulopathy (MCG) which has not previously been reported in the literature in a cat. The proteinuria and HES initially improved while the cat was treated with more traditional medications; however, both the problems persisted for 30 months that the cat was followed subsequently. Previous studies demonstrating the safety and efficacy of imatinib in cats do not report any glomerular injury or significant adverse drug reactions, and the exact cause of this cat's proteinuric nephropathy is uncertain. Nonetheless, the possibility of an adverse drug reaction causing proteinuria should be considered when initiating treatment with imatinib in a cat.

A 6-year-old neutered male domestic shorthair cat was evaluated by the Feline Internal Medicine service at Texas A&M University College of Veterinary Medicine and Biomedical Sciences with a 2-week history of vomiting, diarrhea, and hyporexia. He had no history of previous illness, was up-to-date with monthly flea/tick/heartworm preventatives and was a strictly indoor cat with no other cats in the household.

Upon examination, the cat was bright, alert and responsive, with a body weight of 5.2 kg (11.4 lbs). Temperature (100.2°F), heart rate (162 bpm), and respiratory rate (42 breaths per min) were within normal limits. Thoracic auscultation demonstrated normal bronchovesicular sounds and a regular, synchronous heart beat with no appreciable murmurs or arrhythmias. Abdominal palpation revealed a full urinary bladder but was otherwise unremarkable. A complete blood count (CBC) indicated an eosinophilia (Table 1), but was otherwise unremarkable. Routine serum biochemical parameters were within their reference intervals. Urinalysis showed a concentrated urine with evidence of mild proteinuria. Serum folate was mildly decreased at 5.1 μg/l (reference interval 10–22 μg/dl), cobalamin was within its reference interval, and feline pancreatic lipase immunoreactivity was increased at 13.7 μg/dl (reference interval 2–6.8 μg/dl). Heartworm antigen and antibody tests were both negative. Abdominal ultrasound revealed an enlarged spleen (1 cm in thickness) with normal architecture and parenchyma. Ultrasound guided aspirates of the liver and spleen showed a moderate and a marked eosinophilic infiltrate, respectively.

Table 1.

Pertinent laboratory results.

	T−3	T−2	T−1	T0	T+1	T+2	T+3	T+4
Eosinophils/μl (0–1500×109/l)	8619 (8.62)	3575 (3.58)	938 (0.94)	1490 (1.49)	272 (0.27)	1800 (1.8)	408 (0.41)	153 (0.15)
ALT (26–84 U/l) (0.4–1.4 μkat/l)	84 (1.4)	68 (1.1)	85 (1.4)	6 (0.1)	12 (0.2)	45 (0.8)	56 (0.9)	1236 (20.6)
Albumin (2.5–3.3 g/dl) (25–33 g/l)	3.2 (32)	3.3 (33)	2.9 (29)	2.0 (20)	2.3 (23)	2.7 (27)	3.3 (33)	3.4 (34)
Cholesterol (56–161 mg/l) (1.5–4.2 mmol/l)	128 (3.3)	144 (3.7)	96 (2.5)	246 (6.4)	403 (10.4)	509 (13.18)	493 (12.8)	312 (8.1)
Creatinine (0.8–1.8 mg/dl) (70.7–159.1 μmol/l)	1.3 (114.9)	1.3 (114.9)	1.3 (114.9)	1.7 (150.3)	2.0 (176.8)	1.6 (141.4)	1.2 (106.1)	1.0 (88.4)
Glucose (65–131 mg/dl) (3.6–7.3 mmol/l)	97 (5.4)	92 (5.1)	102 (5.7)	141 (7.8)	99 (5.5)	95 (5.3)	152 (8.4)	241 (13.4)
USG (1.020–1.040)	1.059	N/A	N/A	1.049	1.029	1.027	1.026	1.038
Urine protein	1+	N/A	N/A	4+	4+	3+	3+	4+
UP:C (<0.4)	N/A	N/A	N/A	14.2	7.4	2.8	2.8	6.0

T−3: initial diagnosis of hypereosinophilic syndrome and commencement of imatinib therapy. T−2: 1 week of imatinib therapy. T−1: 24 days of imatinib therapy. T0: 8 weeks of imatinib therapy. Imatinib was discontinued at this time. T+1: 2 weeks post discontinuation of imatinib and post renal biopsy. T+2: 4 weeks post discontinuation of imatinib, methylprednisolone started. T+3: 5 weeks post discontinuation of imatinib. T+4: 9 weeks post discontinuation of imatinib, 5 weeks on methylprednisolone. ALT=alanine aminotransferase; USG=urine specific gravity, UP:C=urinary protein: creatinine.

Routine treatment with fenbendazole (Panacur; Intervet, DE) (50 mg/kg PO q 24 h for 3 days) was administered. Based on the cytologic results, hematologic findings, and clinical signs, the cat was diagnosed with hypereosinophilic syndrome (HES) and treatment with imatinib mesylate (Gleevec; Novartis Pharmaceuticals, NJ) (9.6 mg/kg PO q 24 h) was initiated.

The cat was re-evaluated 1 week after starting imatinib therapy. The absolute eosinophil count was, and he was reportedly improving in overall attitude and appetite. When another recheck was performed 24 days after initiating imatinib therapy, the peripheral eosinophil count was normal and results of a serum biochemical profile were within the reference intervals. At home he was reportedly having intermittent hyporexia, some vomiting, and weight loss (weighing 5.0 kg (11 lbs)) despite a normal attitude and activity level. He was discharged with canned cat food and mirtazapine (Remeron; Organon, USA, a subsidiary of Merck & Co) as an appetite stimulant (0.8 mg/kg PO q 72 h).

The cat was next rechecked after 2 months of imatinib therapy. He continued to have a declining appetite bordering on complete anorexia, was vomiting weekly, and was hiding on occasion. Water consumption and urination remained within normal limits. The CBC showed that the eosinophil count had increased, but there were no other significant hematologic findings. A serum biochemical profile demonstrated an increase in creatinine, hypercholesterolemia, and hypoalbuminemia. Urinalysis showed a USG of 1.049 with proteinuria, hematuria (40–50 red blood cells/hpf), 1–2 squamous epithelial cells/hpf, and 0–1 renal epithelial cells/hpf. The calculated UP:C was 14.2 (reference interval 0–0.4). A urine culture was negative for bacterial growth. These findings were consistent with a proteinuric nephropathy in addition to the HES. Mirtazapine was continued at the previously prescribed dose and the imatinib was discontinued. The cat was started on benazepril (Benazepril (originally Lotensin); Novartis Pharmaceuticals, NJ) (0.5 mg/kg PO q 24 h), and a renal biopsy was scheduled for 3 days later. On the day of the biopsy, the owner reported that the cat's appetite had improved since the imatinib was discontinued. Systolic arterial blood pressures were measured using a Doppler method and were 130, 138, 138, and 148 mmHg. Persistent significant proteinuria was verified in a sample obtained by cystocentesis (UP:C ratio, 19.5). Renal biopsies were obtained under general anesthesia with an automated 18-gauge needle biopsy device (Bard Monopty; Bard Biposy Systems, AZ) using ultrasound guidance. Four cortical tissue cores were retrieved for light microscopic, transmission electron microscopic, and immunostaining evaluations. Two cores that contained approximately 30 glomeruli for evaluation were sectioned at 3 μ, and stained with hematoxylin and eosin (Fisher Scientific, TX) Masson's trichrome (DAKO, Carpinteria, CA), and periodic acid-Schiff (DAKO, Carpinteria, CA) for histologic examination. All glomeruli exhibited similar mild changes characterized by mild mesangial thickening and minimal mesangial hypercellularity (Fig 1). Diffusely tubules contained abundant cytoplasmic lipid vacuoles but otherwise were within normal limits. Neither interstitial fibrosis nor inflammation was present. Ultrastructural evaluation of two glomeruli showed extensive effacement of visceral epithelial cell (podocyte) foot processes (Fig 2). Additionally, many podocytes were swollen, and some contained cytoplasmic lipid and/or residual bodies (Fig 2). Of note, electron-dense deposits suggestive of immune-complex deposition were not found in the capillary walls or mesangium. One glomerulus was available for immunostaining; labeling for IgG and IgA was negative, and there was weak (1+) non-specific labeling for IgM. Taken together, the pathologic changes were judged to be most consistent with a minimal change glomerulopathy (MCG) because of the extensive changes in the visceral epithelial cells (ie, podocytes) accompanied by negligible changes in the rest of the kidney.

Fig 1.

Photomicrographs of a representative glomerulus (exhibiting mild mesangial expansion and minimal mesangial hypercellularity (left: hematoxylin and eosin stain; right: periodic acid-Schiff stain).

Fig 2.

Electron micrograph of a representative glomerulus. There is extensive foot process fusion (black arrows) and cytoplasmic lipid vacuoles within podocytes (white arrowhead).

The cat was next rechecked 2 weeks after discontinuing imatinib. The CBC demonstrated a leukocytosis (27.2×10³/μl: reference interval 5.5–19.5×10³/μl) with a mature neutrophilia (24,752/μl; reference interval 2500–12,500/μl); the eosinophil count was within normal limits (272/μl). Biochemical abnormalities included hypercholesterolemia, elevated creatinine, hypoalbuminemia, and hypokalemia (3.0 mmol/l; reference interval 3.5–5.1 mmol/l). Urinalysis demonstrated a USG of 1.029 and proteinuria (4+ by SSA) as the most significant abnormalities. The UP:C was improved at 7.4. He was continued on mirtazapine and benazepril as previously prescribed, and potassium gluconate powder (Tumil-K; Virbac AH, Fort Worth, TX) (0.2 mEq/kg on food q 12 h) was prescribed for the hypokalemia.

At 4 weeks after discontinuing imatinib therapy, his attitude and appetite were described as cyclic. He was eating a commercial adult maintenance diet and would no longer eat the chicken liver supplement the owner had previously been offering. His weight was continuing to improve and was increasing after reaching its nadir of 4.3 kg (9.4 lbs). Physical examination again was unremarkable. A CBC demonstrated a mild mature neutrophilia (13,860/μl) and an eosinophilia (1800/μl). The most significant abnormality on the serum biochemical profile was hypercholesterolemia. Albumin was within the normal range, and the cat's azotemia had resolved. Urinalysis showed mildly concentrated urine (USG 1.027) and moderate proteinuria (UP:C 2.8). Due to the increase in the eosinophil count, methylprednisolone (Pfizer US Pharmaceuticals, NY) (0.9 mg/kg PO q 12 h) was started; benazepril, potassium gluconate and mirtazapine were continued. Five weeks after discontinuing imatinib the cat no longer needed mirtazapine and had both good activity and appetite. His weight was stable from his previous examination at 4.3 kg (9.4 lbs). Physical examination was again unremarkable. His CBC demonstrated a favorable response to the methylprednisolone administration with a decrease in the eosinophil count to within the normal range. Biochemical abnormalities included mild hyperglycemia and hypercholesterolemia. Albumin continued to increase and was now 3.3 mg/dl and potassium was responding well to supplementation (4.7 mmol/l). Urine specific gravity was 1.026 with a UP:C of 4. The same dose of methylprednisolone was continued until the next recheck (9 weeks after discontinuing imatinib). At this time, the eosinophil count was still within normal limits (153/μl). Serum biochemical profile revealed hyperglycemia, elevated alanine aminotransferase (ALT) activity, and persistent hypercholesterolemia. Urinalysis demonstrated moderately concentrated urine, significant proteinuria, and glucosuria (1000 mg/dl). His diet was changed to a high protein, low carbohydrate diet to try to improve insulin sensitivity and mitigate the hyperglycemia.

Subsequent rechecks demonstrated adequate control of the hypereosinophlia and hyperglycemia. The ALT activity progressively decreased and returned to the normal range within 7 weeks. At this time, the corticosteroid was changed to prednisolone (Watson Pharmaceuticals, Corona, CA) (1 mg/kg PO q 12 h) for management of HES, and benazepril was increased to 0.5 mg/kg PO q 12 h. Potassium gluconate was continued as before.

During subsequent rechecks, the hyperglycemia was effectively controlled with diet and tapering of the prednisolone down to the lowest necessary dose (from 1.1 mg/kg PO q 12 h to 0.21 mg/kg PO q 12 h). His weight improved and stabilized at 5.4 kg (11.8 lbs). The cat was followed for an additional year and 8 months, during which time the proteinuria persisted (UP:C ranging from 3.3 to 6.11) in the absence of azotemia (creatinine 0.7–1.2 mg/dl (61.88–106.08 μmol/l)) and with appropriately concentrated urine (USG 1.037–1.041).

To the authors’ knowledge, this is the first report of MCG in a cat. This disease is a common cause of nephrotic syndrome in human patients, and is most often an adverse drug reaction, toxic, idiopathic, or immunologic in origin. In this cat, it was initially thought that the proteinuria could be secondary to HES, even though it was diagnosed when the peripheral eosinophil count was within the normal range. Despite the absence of immunofluorescence or eosinophilic infiltrate, HES could still be the underlying cause of the minimal change disease (MCD) in this patient. Experimental over-expression of IL-13 in rats induced minimal change-like nephropathy, so interleukin over-expression associated with HES could induce a similar change. ¹ Renal biopsy findings raised the possibility that the glomerulopathy was a consequence of imatinib therapy due to the lack of abnormalities, which is typically how one arrives at a diagnosis of MCD – a diagnosis of exclusion. Immune mediated disease is notoriously difficult to diagnose and is considered a differential for this cat's MCG. MCG was recently reported following the administration of a similar drug (masitinib) to a dog, further supporting the possibility that imatinib triggered the disease in this feline patient. ²

Imatinib mesylate (ST1571: Gleevec; Novartis Pharmaceuticals) is a small protein kinase inhibitor, targeting the tyrosine kinase domain of BCR-ABL, c-KIT, and platelet-derived growth factor (PDGF). ³ It is presently approved by the FDA for people with chronic myelogenous leukemia and gastrointestinal stromal tumors. ^4,5 In addition, it is first line therapy for idiopathic HES in humans. ⁶ Although there are no reports of its use for HES in cats, prior studies have demonstrated its safety in this species and it is currently being evaluated for efficacy against a variety of tumors in both canine and feline patients. ^7,8 The dose for this cat (9.6 mg/kg PO q 24 h) was extrapolated from previously published doses. ⁸ The cat did appear initially to have a positive response to imatinib, with clinical improvement and resolution of the hypereosinophilia 1 month after initiating therapy. Further, the clinical signs of vomiting and anorexia also improved until the cat developed a proteinuric nephropathy.

In a 2008 study evaluating the safety of masitinib in dogs, 7.5% of dogs had adverse reactions involving the kidneys, including renal failure, glomerulonephritis, nephrotic syndrome, and/or proteinuria. Only 50% of those dogs recovered from their renal injury. ⁹ The proposed mechanism of action behind the glomerular injury was masitinib-induced inhibition of PDGF receptors. These receptors have been demonstrated within glomeruli of primates and are presumed to be present in canine glomeruli. ¹⁰ It is theorized that canine glomerular cells, particularly podocytes have altered function due to PDGF inhibition resulting in proteinuria. Additional recent studies on the safety of masitinib in dogs have not shown any observation of the previously reported nephrotoxicity, however, no urine was examined as a component of their assessment in this study. ¹¹ Masitinib, although in the same class of drug, is not the same as imatinib and one cannot assume that they will have similar adverse effects.

In a classic drug-induced MCG, the proteinuria would resolve when the drug is discontinued as was observed in the recent report of MCD in a dog. ² This particular cat experienced partial, though not complete, resolution of proteinuria (the UP:C decreased from 19.5 to 2.8) when the imatinib was withdrawn. The underlying etiology for the incomplete resolution of the proteinuria is unknown and a repeat renal biopsy would be necessary to determine if the excessive proteinuria could have caused progression to glomerulosclerosis or be related to continued glucocorticoid administration. The impact of glucocorticoids on proteinuria in the cat has not been reported, but a study in dogs indicated that 2.2 mg/kg resulted in a maximal UP:C of 1.27±1.02 by day 28. ¹²

If this cat's minimal change nephropathy was in fact drug-induced, it is possible that the mirtazapine was a contributory factor. The dosage of mirtazapine given to this patient (0.8 mg/kg PO q 72 h) was based on anecdotal reports as there is little published data on the use of this drug in cats. Commonly documented side effects of this drug in people are behavioral/neurologic and range from myoclonus, hyperreflexia to overactivity/agitation, fever, and hypertonia with no renal adverse events having been reported. ^13,14 Excessive vocalization seems to be the most reported anecdotal side effect of mirtazapine in cats. As the proteinuria improved whilst the cat was still receiving mirtazapine, it seems unlikely that this drug was causative.¹⁵

MCD is a common cause of nephrotic syndrome in humans. It is described uncommonly in the dog and to the authors’ knowledge has never been reported in the cat. ¹ This reinforces the need for thorough evaluation of renal biopsies with not only light microscopy, but also immunofluorescence and electron microscopy for complete evaluation. Imatinib effectively managed HES in this patient for a period of 8 weeks. Although a direct causal relationship between the imatinib and development of MCG was not proven, as the cat's biochemical abnormalities improved but did not completely resolve with discontinuation of imatinib, development of an MCG should be considered a potential complication when considering imatinib as a treatment option for felines, or potentially as a complication of HES.