Presumed masitinib-induced nephrotic syndrome and azotemia in a dog

Abstract

Masitinib mesylate is a tyrosine-kinase inhibitor approved for the treatment of nonresectable or recurrent, Grade 2 or 3 mast cell tumors in dogs. This report describes nephrotic syndrome and acute kidney injury attributed to masitinib and illustrates the need for regular monitoring of serum creatinine concentration, urinalysis, and urine protein:creatinine ratio during its use.

Masitinib mesylate (Kinavet, AB Science, Short Hill, New Jersey, USA) is a tyrosine-kinase inhibitor that has been approved for the treatment of nonresectable or recurrent, Grade 2 or Grade 3 mast cell tumors in dogs (1–10). The primary effect of masitinib is selective and potent inhibition of c-KIT-dependent cell proliferation. Masitinib plays a role in the inhibition of platelet-derived growth factor receptor (PDGFR)-dependent cell proliferation and Src family kinases such as LYN, a contributor to IgE-induced mast cell degranulation in vitro (2,5,7,11). The effect of masitinib on renal function is theorized to be a direct inhibition of c-KIT on the renal tubules or on the glomerular cells that express PDGF receptors (1,7). Reported renal-related side effects of masitinib include proteinuria, nephrotic syndrome, and renal failure (2). To the authors’ knowledge, this is the first report describing a severe case of Masitinib-related nephrotic syndrome (NS) and acute kidney injury (AKI) with complete recovery of renal function based on biochemistry and urinalysis parameters.

Case description

A 3-year-old, 29.8 kg, intact female Weimaraner dog was presented to the University of Minnesota Veterinary Medical Center (VMC) for evaluation of acute onset of vomiting and abdominal distension of 1 wk duration. A mass had been noted on the left cheek 217 d prior to presentation. A Grade 1 mast cell tumor was diagnosed through incomplete surgical debulking with histopathology on day −92. Several weeks later (Day −77), treatment was initiated with masitinib at a dose of 4.4 mg/kg body weight (BW), PO, q12h.

A complete blood (cell) count (CBC) and serum chemistry performed by the referring veterinarian for an unrelated surgery prior to beginning the masitinib therapy (Day −337), revealed abnormalities that were consistent with mild dehydration (Table 1). A CBC and serum chemistry performed 1 mo (Day −30) after initiation of the medication revealed no significant abnormalities. The day before presentation (Day −1) to the VMC, the dog had been examined by her referring veterinarian for lethargy, inappetance, polydipsia, and abdominal distension of 1 wk duration. At that time, the serum chemistry results indicated panhypoproteinemia (Table 1) on an in-house chemistry analyzer and the referring veterinarian discontinued masitinib and recommended a high protein diet.

Table 1.

Changes in biochemistry values over time

Day	CREA 53 to 141 μmol/L	BUN 3.2 to 11.0 mmol/L	TP 50 to 69 g/L	Albumin 27 to 37 g/L	Globulin 17 to 35 g/L	Sodium 145 to 153 mmol/L	Chloride 109 to 118 mmol/L	Phosphorus 1.07 to 2.2 mmol/L	Bicarbonate 15 to 25 mmol/L	Cholesterol 3.7 to 9.66 mmol/L
−337	61.88	3.93	78	45	32	138	NA	1.26	NA	NA
−30a	61.88	4.28	68	42	26	142	NA	1.42	NA	NA
−1b	97.24	6.78	34	21	13	130	NA	2.45	NA	NA
1c	114.92	7.14	35	9	26	144	120	1.65	11.8	11.99
3	150.28	17.14	25	< 15	NDd	140	126	2.07	7.6	9.14
4e	256.36	25.35	38	10	27	141	122	3.13	6	10.08
5	344.76	32.84	36	11	24	140	120	2.71	9.8	8.62
6	362.44	32.48	36	11	24	142	122	2.68	10.8	8.81
9	316.24	26.76	37	12	25	141	122	2.61	9.5	10.77
16	97.24	9.28	44	16	28	143	111	1.65	22.9	12.71
30	70.72	4.64	58	27	31	145	112	1.20	24	9.19
44	70.72	5.71	60	30	30	145	111	1.20	22.7	6.99
196	70.72	6.07	63	35	28	145	111	1.03	22.4	5.96

Masitinib was initiated on day −77.

^aA CBC and serum chemistry were performed within 1 mo of initiation of therapy (day −30).

^bMasitinib was discontinued on day −1.

^cThe subcutaneous mass on the left check was noted in the same location as the previously incompletely excised mast cell tumor.

^dThe globulin value was not detectable on the chemistry analyzer.

^eThe catheter site infection was observed.

CREA — creatine; BUN — blood urea nitrogen; TP — total protein; NA — not available.

The following day (Day 0), the dog was presented to the University of Minnesota emergency service for vomiting and lethargy of 6 h duration. Physical examination revealed moderate submandibular edema and profound abdominal distension. As a consequence, packed cell volume, total plasma protein, point-of-care chemistry analysis (Chem8+ iStat; Abaxis, Union City, California, USA), 4DX SNAP test (IDEXX, Westbrook, Maine, USA), abdominal radiographs, abdominal FAST scan (Focused Assesment with Sonography for Trauma Scan) and abdominocentesis were performed. The total plasma protein was low at 48 g/L; however, the limited chemistry panel was unremarkable. Serological tests for Diroflaria immitis, Anaplasma phagocytophilum, Ehrlichia canis, and Borrelia burgdorferi were negative. Abdominal radiographs revealed poor serosal detail due to abdominal effusion, and the FAST scan confirmed a moderate to severe abdominal effusion. Cytology of the abdominal fluid revealed a transudate with protein (< 25 g/L), and a total nucleated cell count of 680 cells/μL. Systolic blood pressure was 120 mmHg. Upon admission into the Intensive Care Unit, a colloid osmotic pressure was determined to be decreased at 6.5 mmHg [reference interval (RI): 19.95 to 22.05 mmHg], and a metoclopramide (Reglan; Hospira, Lake Forrest, Illinois, USA) constant rate infusion (CRI) and a subcutaneous injection of Maropitant (Cerenia; Zoetis, Madison, New Jersey, USA) were administered to address the vomiting.

The next morning (Day 1), physical examination revealed an approximately 2 cm diameter movable, soft, subcutaneous mass on the left cheek, a mild amount of dependent subcutaneous edema on the ventral thorax, and abdominal distension. The subcutaneous mass on the left cheek was consistent with recurrence of the previously diagnosed mast cell tumor. A CBC, serum chemistry, abdominal ultrasound, and thoracic radiographs were performed. A serum chemisty panel revealed hypoalbuminemia, hypobicarbonemia, and hypercholesterolemia (Table 1). The urine sample obtained on admission was not evaluated and reported until Day 3. This urinalysis revealed isothenuria, hematuria, proteinuria, pyuria, and bacteruria (Table 2). The urine protein:creatinine ratio (UPC) was significantly elevated at 36.3 (normally < 0.2). Abdominal ultrasound revealed marked abdominal effusion, edema of the gall bladder wall, and subcutaneous edema. Thoracic radiographs and a CBC were unremarkable. Colloid osmotic pressure on Day 2 remained low at 7.6 mmHg (RI: 19.95 to 22.05 mmHg). Treatment for Days 1 and 2 included Hetastarch (Hydroxyethyl starch; Teva Parenteral Medicines, Irvine, California, USA) and a proton pump inhibitor (Pantoprazole; Protonix, Pfizer, Madison, New Jersey, USA). Dexamethasone-SP (Dexamethasone Sodium Phosphate; Vedco, St. Joseph, Missouri, USA) and diphenhydramine (Benadryl; Parkedale Pharmaceuticals, Rochester, Michigan, USA) were administered once intravenously followed by oral diphenhydramine. Diphenhydramine and dexamethasone-SP were administered to minimize increased vascular permeability secondary to histamine from the mast cell tumor. Maropitant was continued throughout the course of hospitalization.

Table 2.

Urinalysis and urine protein:creatinine values over time

Day	USG	pH	Occult blood	Protein	WBC	Bacteria	UPC
0a	1.013	7.5	2+	3+	5 to 20/HPF	Many rods/HPF	36.3
4	1.021	6.5	1+	3+	occasional	none	NA
6	1.017	6.5	2+	3+	5 to 20/HPF	none	23.7
16	1.013	6.5	1+	2+	occasional	none	8.3
30							0.1
44							0.1
196	1.025	7.5	Trace	Negative	Negative	Negative	0.1

^aThe day 0 sample was not reported until day 3.

USG — urine specific gravity; WBC — white blood cells; UPC — urine protein:creatinine ratio, which is normally < 0.2, urine samples were collected by cystocentesis on days 4 to 196; HPF — high-powered field; NA — not available.

On the third day, a moderate amount of a subcutaneous edema of the vulva, perianal region, and hind limbs was present. Pyrexia of 40.6°C and a mild catheter site inflammation were discovered that afternoon. Purulent debris exuded from the catheter site during removal and the body temperature returned to normal (37.9°C) several hours after removal of the catheter and addition of antibiotics. Cytologic examination of the purulent debris revealed a marked septic neutrophilic inflammation with cocci bacteria. A serum chemistry, urine sodium, and Rocky Mountain spotted fever titer revealed panhypoproteinemia, progressive hypobicarbonemia, and azotemia (Table 1). Urine sodium concentration determined on a urine sample obtained on Day 3 was elevated at 59 mmol/L (prerenal: < 20 mmol/L) possibly indicating active renal tubular dysfunction or a physiologic response post hetastarch therapy (14). The Rocky Mountain spotted fever titer was negative. Ampicillin sublactam (Pfizer, New York, New York, USA), 22 mg/kg body weight (BW), q8h, and enrofloxacin (Bayer, Shawnee Mission, Kansas, USA), 10 mg/kg BW, q24h, were initiated and Prednisone (Prednisone; Teva Parenteral Medicines, Irvine, California, USA), 0.5 mg/kg BW, q24h was continued to prevent mast cell degranulation. Hetastarch was discontinued due to the progression of the subcutaneous edema and poor colloid osmotic pressure response suggesting possible overfill. Ondansetron (Zofran; Wockhart, Mumbai, India) and famotidine (Pepcid; Westward Pharmaceutical, Eatontown, New Jersey, USA) were administered for the vomiting prior to presentation.

The catheter site became severely inflamed and painful on Day 4. An ultrasound of the limb revealed 3 separate pockets of fluid; culture and sensitivity testing of the fluid grew Staphylococcus intermedius group resistant to ampicillin and penicillin, but susceptible to enrofloxacin. Three separate blood cultures revealed no bacterial growth after 5 d. A CBC revealed an inflammatory leukogram and a mild nonregenerative anemia: leukocytosis 22 470/μL (RI: 3880 to 14 570/μL), neutrophilia 20 220/μL (RI: 2100 to 11 200/μL), with a regenerative left shift 220/μL (RI: 0 to 130/μL), lymphopenia 450/μL (RI: 780 to 3360/μL), and a monocytosis 1570/μL (RI: 0 to 1200/μL). A nonrengenerative anemia was present with a hematocrit of 34% (37.5% to 60.3%), red blood cell concentration of 4.96 × 10⁶/μL (RI: 5.44 to 8.79 × 10⁶/μL), and a hemoglobin concentration of 118 g/L (RI: 133 to 207 g/L) were noted. The serum chemistry revealed progression of the renal dysfunction with progressive elevations in blood urea nitrogen (BUN), creatinine, and phosphate, worsened hypobicarbonemia, and static serum albumin concentration (Table 1). Urinalysis revealed a persistent proteinuria with occasional white blood cells, but no bacteria (Table 2). A urine culture was sterile. The prothrombin time (PT) was within normal limits, but the activated partial thromboplastin time (aPTT) was slightly prolonged at 21.3 s (RI: 9.8 to 14.6 s). Venous blood gas (EC7+ iStat; Abaxis) revealed a metabolic acidosis with respiratory compensation: pH of 7.095 (RI: pH 7.3 to 7.47), hypobicarbonatemia 10 mmol/L (RI: 17.8 to 27.2 mmol/L), pCO, 32.5 mmHg (RI: 28.9 to 44.4 mmHg), and total CO₂ 11 mmol/L (18.6 to 28.4 mmol/L). Metabolic acidosis was treated with sodium bicarbonate (Hospira). Massage and warm compresses were initiated to improve blood and lymphatic circulation to the affected limb. A fentanyl patch (Duragesic Patch; Noven Pharmaceutical, Miami, Florida, USA) was applied for the pain, and clinicare liquid diet (CliniCare; Abbott, Abbott Park, Illinois, USA) was administration through a nasoesophageal feeding tube.

Clinical improvement of the catheter site infection was noted on Day 5. A serum chemistry profile revealed further progression of the azotemia, and a relatively static hyperphosphatemia, hypoalbuminemia, and hypobicarbonatemia (Table 1). The point-of-care venous blood gas analysis revealed improvement in the metabolic acidosis with a pH 7.137, hypobicarbonatemia 10.2 mmol/L (RI: 17.8 to 27.2 mmol/L), and pCO₂ 37.1 mmHg (RI: 28.9 to 44.4 mmHg). Pantoprazole and Trimethoprim sulfamethoxazole (SMX-TMP; Amneal Pharmaceutical, Hauppauge, New York, USA) were added to the treatment protocol, and ampicillin sulbactam was discontinued based on the culture and sensitivity results of the catheter site infection.

On Day 6, the serum creatinine concentration continued to rise; however, the other blood values remained relatively stable (Table 1). The UPC ratio remained elevated (Table 2). The dog’s clinical signs appeared to be improving, and she was discharged from the hospital the same afternoon.

The dog was monitored closely on an outpatient basis and renal profiles were repeated on days 9, 16, 30, and 44. The UPC ratio values were repeated on days 16, 30, and 44 and a urinalysis was performed on day 16. Three days after discharge (Day 9), a renal profile revealed mild improvement in serum creatinine, urea nitrogen, phosphorus, and bicarbonate concentrations while serum albumin concentration remained static. Hypercholesterolemia worsened, most likely attributable to recent feeding (Table 1). One week later (Day 16), the azotemia had completely resolved and both hypoproteinemia, and hypoalbuminemia had improved. The UPC ratio was markedly improved but proteinuria persisted (Table 2). By day 30, the renal and electrolyte values had returned to pre-masitinib levels and the UPC ratio was normal (Table 1). These values were similar on days 44 and 196.

Discussion

The Weimaraner dog in this case report had laboratory and physical examination findings consistent with nephrotic syndrome (NS) and concurrent acute kidney injury (AKI) presumed secondary to masitinib administration. Masitinib mesylate is labeled for use in dogs for Grades 2 or 3 nonresectable mast cell tumors at a dose of 12 mg/kg BW per day (2,4). In this case, masitinib was used at a dose lower than recommended, and in an extralabel fashion in that the mast cell tumor was Grade 1. The noteworthy and not previously reported aspect of this case is the recovery of normal kidney function with no apparent long-term sequellae despite masitinib-associated severe nephrotic syndrome and concurrent IRIS AKI Grade II. This case demonstrates that dogs affected by severe masitinib-associated nephrotic syndrome with concurrent AKI can recover and return to normal levels of kidney function without long-term complications when treated by withdrawal of masitinib and supporting therapy.

Nephrotic syndrome is defined as the presence of hypoalbuminemia, proteinuria, extravasulcar fluid accumulation, and hypercholesterolemia. Nephrotic syndrome is seemingly uncommon in dogs, with the median number of dogs diagnosed with NS at tertiary institutions reported to be 0.5 every 12 mo (12). Hypoalbuminemia and proteinuria occur due to the loss of selective permeability of the glomerulus. The hypoalbuminemia may cause secondary hypercholesterolemia due to hepatic biosynthesis upregulation, changes in plasma viscosity, or as a compensatory mechanism to low plasma oncotic pressure (12,13). Ascites occurs in approximately 75% of canine NS cases. Pitting edema and subcutaneous fluid, the most common finding in people with NS, is noted in 60% of dogs. The ascites fluid is characterized as a pure transudate with a nucleated cell count of less than 1500 cells/μL and a protein concentration < 25 g/L (12,13). It is possible that masitinib has an undescribed direct effect on peripheral blood vessels. Edema, primarily periorbital, was noted in 26% of humans during a masitinib trial for rheumatoid arthritis. In a sizable proportion of patients, the edema persisted and was thought to be secondary to PDGFR inhibition in the periorbital region (11). Periorbital edema has been described as a possible complication of masitinib in humans and it may be a contributing factor to the subcutaneous edema in the dog in the present case.

The progressive azotemia in this case is consistent with AKI, which may be attributable to the drug’s effect on the glomerulus, renal tubules, or both. Other causes of AKI such as pyelonephritis and hypoperfusion are less likely, given the continued progressive azotemia despite adequate volume support and broad spectrum antibiotics. In the safety and efficacy study published by Hahn et al (4), renal disorders were reported in 12 out of 161 dogs (7.5%) following treatment with masitinib, compared to 2 dogs (4.9%) in the placebo group. Renal dysfunction in this population of dogs was characterized as glomerulonephritis, renal failure, nephrotic syndrome, and/or proteinuria. Renal biopsy with electron microscopy was not performed in any of the dogs with noted renal disorders; therefore, the type of glomerular or tubular lesion was not definitively diagnosed in this study. All of the dogs affected with renal dysfunction had baseline BUN and creatinine values that were high/normal to mildly elevated (1,2). Three of the 12 dogs were euthanized and 6 dogs recovered without sequelae (1). In contrast with the findings in the safety and efficacy study, the dog in the current case had a normal creatinine prior to the development of AKI. A safety trial of masitinib mesylate in cats documented significant elevations in serum creatinine concentrations and a corresponding drop in serum albumin concentrations from baseline levels (15). Although changes in creatinine and serum albumin concentrations were noted from baseline, neither value appeared outside reference ranges in the population of study cats. Furthermore, 10% of the cats in this study experienced reversible proteinuria (15).

Renal complications associated with masitinib have been reported in the veterinary literature. Minimal change nephropathy secondary to masitinib administration was described in a giant schnauzer (7) and in a Labrador retriever (1). Clinical findings in the schnauzer included progressive panhypoproteinemia, and proteinuria without azotemia. The Labrador presented with vomiting, and abdominal pain, which progressed to severe subcutaneous edema and oligouric renal failure. Light microscopy on both dogs’ kidneys postmortem revealed podocyte enlargement, mild thickening or splitting of the basement membrane in the Bowman’s capsule, and slight swelling of the proximal tubular epithelium. Lesions observed by electron microscopy in both dogs confirmed podocyte effacement, swelling, and villous transformation. Patchy interstitial inflammation and multiple microthrombi were noted upon cross section of the Labrador’s kidney. While the Schnauzer’s nephrotic syndrome clinically resolved, the Labrador was ultimately euthanized due to increasing renal dysfunction (1,7).

Interpretation of the UPC ratio in this case is complicated by the bacteriuria and pyuria detected in the first urine sample. The urinary tract infection (UTI) is a possible cause for proteinuria. In our case, the method of the first urine collection was unclear and, while it was collected on day 1, it was not analyzed for 3 d. A meta-analysis of 80 human studies examining associations between proteinuria and asymptomatic UTI found that in only 10 studies did UTI have a significant effect on proteinuria (16). Although significantly elevated UPC ratio values have been reported in several dogs with experimentally induced cystitis (17), a prospective study of the effect of pyuria and bacteriuria in dogs revealed no correlation between the degree of pyuria and the magnitude of increase in UPC ratio. For 39 dogs with 6 to 20 white blood cells/high-powered field (HPF), similar to this dog, the UPC ratio ranged from 0 to 0.55 (median: 0.16). The UPC ratio values in 26 dogs with observed bacteriuria ranged from 0 to 1.4 (median: 0.2) (18). Similar to our case, the Labrador reported to have masitinib-associated minimal change nephropathy had 5 to 10 WBC/HPF, 4+ protein, 3+ blood, and a UPC ratio of 12 prior to euthanasia (1). Therefore, it is likely that the markedly elevated UPC ratio observed in the current case was not a consequence of bacterial UTI. It is possible that this dog’s AKI may be a consequence of pyelonephritis; however, pyelonephritis does not adequately explain the nephrotic syndrome in this dog. Further, the association between AKI and masitinib is well-established in the efficacy and safety study and previous case reports. In addition, there was no evidence of pyelonephritis in this dog.

The catheter site infection observed in this case is likely attributed to immunosuppression secondary to kidney dysfunction and/or administration of corticosteroid therapy to prevent mast cell degranulation. However, secondary infections have been well-documented in humans with NS. Humans have an up to 13-fold increase in risk for the development of bacterial infections and sepsis (19,20). However, this complication has yet to be clearly associated with NS in the veterinary literature.

Most dogs in the study by Hahn et al (4) and the dog in the minimal change nephropathy case report (7) recovered fully with no long-term sequelae. Despite the severity of the azotemia and NS, the dog in this case report recovered rapidly, and without relapse or long-term sequelae after discontinuation of the medication. Based on the number of dogs noted to have elevations in renal values and/or proteinuria in the study by Hahn et al (4), it seems prudent to recommend serial monitoring of serum chemistry, urinalysis, and UPC ratio for all dogs receiving masitinib mesylate. Significant elevations in renal values and/or proteinurina dictate discontinuation of the medication and subsequent monitoring of renal function. Serial UPC ratio may be an early marker for renal dysfunction in dogs treated with masitinib (6). When this dog was initially presented to the referring veterinarian, she had a serum creatinine above her previously observed values but not greater than the reference range. Elevations in serum creatinine above baseline may warrant discontinuation of masitinib, or at the very least increased monitoring, even if the serum creatinine concentration remains within reference range. CVJ