Risk factors associated with progressive increases in serum creatinine concentrations in cats with cancer receiving doxorubicin

Abstract Background Azotemia occurs in cats administered doxorubicin, but risk factors have not been explored. Objective To determine incidence of progressive increases in serum creatinine concentration in cats with cancer receiving doxorubicin in single or multiagent chemotherapy protocols and associated risk factors. Animals Seventy cats with cancer receiving doxorubicin. Methods A retrospective study (2007‐2017) of cats with indices of kidney function recorded before and after doxorubicin administration was reviewed. Cats diagnosed with kidney injury because of known etiologies other than possible doxorubicin toxicosis were excluded. Variables were compared to identify risk factors. Results Mean age (±SD) was 10.9 years (±3.2). Cancer types included lymphoma (n = 36), sarcoma (n = 19) and carcinoma (n = 14). Chronic kidney disease was present in 29/70 (41%) cats before receiving doxorubicin. Of 70 cats, 24 (34%) developed an increase in serum creatinine concentration ≥0.3 mg/dL and 10 (14%) had an increase ≥50% from baseline. Mean time to increases in serum creatinine concentration ≥0.3 mg/dL from first administration of doxorubicin was 119.3 days (±89.7), with mean 2.8 (±1.2) doses administered. Neutropenia or anemia during chemotherapy and number of radiation therapy treatments under general anesthesia were risk factors for increases in serum creatinine concentration (P < .05). Cats receiving single agent doxorubicin had a higher likelihood of an increase in serum creatinine concentration ≥0.3 mg/dL from baseline than cats receiving CHOP‐based chemotherapy protocols (OR 20.0, 95% CI 2.9‐100). Conclusions and Clinical Importance Progressive increases in serum creatinine concentration from baseline were common in cats receiving doxorubicin and associated risk factors were identified.


| INTRODUCTION
Doxorubicin is an anthracycline antibiotic that is used in both single agent and multiagent chemotherapeutic protocols for treatment of a variety of cancers in cats. 1 Doxorubicin-associated nephrotoxicosis, as defined by development of overt azotemia, occurs in cats, rabbits and pigs. [2][3][4][5] In an experimental study of healthy, young adult cats administered doxorubicin at a dose of 30 mg/m 2 every 21 days to a cumulative dose of 300 mg/m 2 , serum creatinine concentrations increased outside the reference interval in 2/6 cats and serum creatinine clearance decreased significantly in all cats during the study period. For the 2 cats that became azotemic, serum creatinine concentration increased from 1.0 and 1.1 mg/dL to 1.6 and 2.6 mg/dL, respectively. These cats had mild to moderate mesangial matrix thickening, tubular cell necrosis and tubular regeneration, with some cats also having mild to moderate nonsuppurative interstitial nephritis. 2 Development of azotemia occurs in cats receiving doxorubicin for treatment of various cancers, 6 with suggestions that this is in part associated with the cumulative dose. 6,7 In cats with vaccine-associated sarcoma, 5/55 (9%) cats became azotemic a median of 8 months after beginning single agent doxorubicin chemotherapy. 6 Two of 17 (12%) of cats with several different cancer types became azotemic after receiving >100 mg/m 2 cumulative doxorubicin dose (combined with cyclophosphamide). 7 Ten of 60 (17%) cats receiving doxorubicin based on either bodyweight or body surface area became azotemic, though there was no difference with regard to BUN and serum creatinine concentrations between groups. 9 In humans, doxorubicin is not associated with nephrotoxicosis, however, other chemotherapeutic agents, including cisplatin, mitomycin C, gemcitabine and ifosfamide, can be nephrotoxic. 9 Several patient-related factors increase the risk and severity of drug-induced kidney injury. Risk factors include age and female sex as both are associated with decreased muscle mass and total body water and therefore likely an underrepresentation of pre-existing kidney dysfunction, as defined by serum creatinine concentration, before administration of chemotherapy. Intravascular volume depletion from vomiting, diarrhea and diuretics also increases the risk of drug-induced nephrotoxicosis.
Cancers such as multiple myeloma, lymphoma and leukemia and diabetes mellitus are associated with increased risk of nephrotoxicosis. 9 While doxorubicin-induced nephrotoxicosis occurs in cats, risk factors for its development have not been thoroughly evaluated. In addition, previous studies have defined decreased kidney function by serum creatinine concentration outside of laboratory reference intervals, which identifies disease late in its course after considerable injury and function loss have occurred. The aims of this retrospective study were to determine the incidence of early loss of kidney function (as defined by ≥0.3 mg/dL increase in serum creatinine concentration) in cats with cancer receiving doxorubicin in single or multiagent chemotherapy protocols and to determine risk factors associated with these progressive elevations in serum creatinine concentration. We hypothesized that pre-existing chronic kidney disease, radiation therapy, recent anesthesia or surgery and cumulative dose of doxorubicin would be risk factors for increasing serum creatinine concentrations in cats receiving doxorubicin. Cats with known or suspected renal lymphoma were also excluded.

| MATERIALS AND METHODS
Renal lymphoma was defined as any cat with cytologic or necropsy confirmation of renal lymphoma or cats where serum creatinine concentration improved with initiation of chemotherapy and where serum creatinine concentration worsened when lymphoma relapsed. Cats were additionally excluded if a cause of kidney injury (eg, ureteral obstruction) other than possible doxorubicin toxicosis was identified or if serum creatinine concentration increased by ≥0.3 mg/dL from baseline before any administration of doxorubicin. Cats were monitored for up to 6 months after completion of the chemotherapy protocol. Indices of kidney function (including serum creatinine and BUN concentrations) were assessed during and after chemotherapy protocols at the discretion of the attending clinician. A sustained increase in serum creatinine concentration by ≥0.3 mg/dL from baseline at any time point after 1 or more doses of doxorubicin was considered to indicate kidney injury, as defined by the International Renal Interest Society (IRIS) acute kidney injury (AKI) guidelines. 10 This value was selected to increase detection of decreased renal function, particularly since many cats with cancer experience weight loss, muscle mass loss or both 11 and muscle mass is recognized to impact serum creatinine concentration. In order to be defined as a sustained increase, serum creatinine concentration had to be increased on more than 1 sequential measurement by ≥0.3 mg/dL from baseline. This was chosen to exclude cats with transient increases such as from prerenal causes. Acute kidney injury was also assessed using the Veterinary Cooperative Oncology Group Common Terminology Criteria for Adverse Events (VCOG-CTCAE) v1.1. 12 The time at which a sustained increase in serum creatinine concentration ≥0.3 mg/ dL from start of chemotherapy was recorded. Staging of chronic kidney disease (CKD) was based on IRIS guidelines 13 using clinicopathologic and, where available, ultrasonographic data; symmetric dimethylarginine (SDMA) was not used for staging in this cohort of cats as it was not regularly available. Specifically, cats were diagnosed with IRIS stage 1 CKD where serum creatinine concentration was <1.6 mg/dL, but there were ultrasonographic changes consistent with CKD such as decreased kidney size, irregular contour, increased echogenicity or decreased corticomedullary differentiation, with or without decreased urine concentration (urine specific gravity <1.035). 13,14 Signalment (including breed, sex, neuter status and age at initiation of chemotherapy) was recorded. Where available, feline immunodeficiency virus (FIV) antibody and feline leukemia virus (FeLV) antigen status were noted. Comorbid disorders known to be risk factors for AKI in humans undergoing chemotherapy or cats were recorded, including diabetes mellitus, urinary tract infection and hyperthyroidism. 9 Table 1. Of 70 cats, 24 cats (34%) were neutropenic at least once during their chemotherapy protocol. Thirty-one cats (44%) were anemic with hematocrit <30% during their chemotherapy protocol, with 11 of these cats having a hematocrit <25%.   Between 9% and 17% cats receiving doxorubicin become azotemic based on traditional evaluation of serum creatinine concentration outside of its reference interval, without consideration of muscle loss. [6][7][8] The number of cats reported in the present study experiencing increases in serum creatinine concentration both above their baseline (34%) and above 1.6 mg/dL (26%) is higher than those reported previously; this partly reflects that increases in serum creatinine concentration within the reference interval were considered, rather than solely development of azotemia. Serum creatinine concentration is not a sensitive indicator of kidney injury or early decreased renal function as it does not increase beyond the reference interval until approximately 75% of total renal mass is lost. However, as utilized in this study, individualizing evaluation by using each cat's serum creatinine concentration as a baseline and then trending serum creatinine concentration over time improves sensitivity. In dogs, use of serum creatinine concentration trending has allowed detection of median 27% decrease in glomerular filtration rate, as compared to an estimated 75% loss before overt azotemia outside of reference ranges develops. 18 In addition, in human patients with pre-existing CKD who experience AKI, use of absolute increases in serum creatinine concentration over baseline can allow for early AKI diagnosis. 19 This is relevant in populations of middle-aged to older cats, where CKD occurs commonly 20 and emphasized in the present study where 41% of cats had IRIS stage 1 and early stage 2 CKD. An absolute increase in serum creatinine concentration above baseline was therefore selected to improve detection of cats with kidney injury and subsequent decreased glomerular filtration rate.
In addition, considering that many cats with cancer experience weight loss, muscle mass loss or both 11  this end-organ may be exposed to higher or more prolonged concentrations of drug. Anemia might be a risk factor for increases in serum creatinine concentration from baseline because of hypoxic injury to the kidney, particularly when combined with nephrotoxic drug administration, though could also reflect more severe concurrent disease.
Both older age and the presence of CKD increase the risk of drug-induced AKI in humans. 9 This was not apparent in the present feline population. Case selection bias might have affected the impact of CKD on doxorubicin-associated nephrotoxicosis as cats with IRIS stage 3 and 4 CKD are generally not prescribed doxorubicin at our institution because of concern for nephrotoxicity. The lack of cats with IRIS stage 3 or 4 CKD could have resulted in underestimation of the risk of doxorubicin-associated nephrotoxicosis in the study population. However, it is also possible that cats with IRIS stage 3 or 4 CKD do not have higher risk of nephrotoxicosis compared to other cats, meaning that doxorubicin is being unnecessarily withheld.
Both the number of radiation therapy treatments and the type of chemotherapy protocol affected the risk of increases in serum creatinine concentration from baseline in cats in this study. The number of radiation therapy treatments was linked to a small increased risk for serum creatinine concentration increases from baseline, possibly because of the number of anesthetic events, as general anesthesia can lead to ischemiaassociated kidney injury. 24  reporting of BCS is recommended to assist in interpretation of changes in serum creatinine concentration from baseline in future studies.

CONFLICT OF INTEREST DECLARATION
Robert Rebhun serves as Associate Editor for the Journal of Veterinary Internal Medicine. He was not involved in review of this manuscript.

OFF-LABEL ANTIMICROBIAL DECLARATION
Authors declare no off-label use of antimicrobials.

INSTITUTIONAL ANIMAL CARE AND USE COMMITTEE (IACUC) OR OTHER APPROVAL DECLARATION
Authors declare no IACUC or other approval was needed.

HUMAN ETHICS APPROVAL DECLARATION
Authors declare human ethics approval was not needed for this study.