Inherent P-glycoprotein deficiency and macrocyclic lactone toxicity in an Australian Maine Coon cat homozygous for ABCB1 1930_1931del TC

Abstract

Case summary

A 9-month-old castrated male Maine Coon cat developed tongue protrusion, mydriasis, generalised weakness and ataxia after routine application, as per label guidelines, of NexGard Spectra (Boehringer Ingelheim; active ingredients praziquantel 83 g/l, esafoxolaner 12 g/l, eprinomectin 4 g/l). The cat was managed conservatively for suspected macrocyclic lactone toxicity and made a complete recovery over 3 months. Further testing identified the cat was homozygous for ABCB11930_1931del TC, a recently identified variant for encoding for the drug transporter molecule P-glycoprotein (P-gp). Cats who are homozygous for ABCB11930_1931del TC have a non-functional P-gp. Because of this change, affected cats have an increased risk of neurotoxicity of macrocyclic lactone and other P-gp substrate drugs, as seen in this case.

Relevance and novel information

 This is the first documented case of a cat homozygous for ABCB11930_1931del TC in Australia with macrocyclic lactone toxicity after administration of an eprinomectin-containing product at label doses. Further awareness of this genetic variant and implications in use of substrate drugs, including new generation macrocyclic lactones, are required to improve the safety of these products in the general cat population. This includes the development and promotion of commercially available screening tests to facilitate personalised treatment advice.

Introduction

P-glycoproteins (P-gps) are critical drug-transporter molecules encoded by the ABCB1 gene. They are present in mammalian tissues, including brain capillary endothelial cells, biliary canaliculi, intestinal mucosa and renal tubular cells. When functional, they limit exposure and enhance the excretion of substrate drugs, including neurotoxins such as macrocyclic lactones. ¹ A P-gp deficiency, which can be inherent or acquired, leads to an accumulation of potentially harmful substrate drugs, increasing the risk of adverse drug reactions.^{2 –4} The ABCB119310_1931del TC variant has only recently been characterised in cats, and when homozygous for this variant, results in a non-functional P-gp.

Known P-gp substrate drugs in cats include macrocyclic lactones (eprinomectin, selamectin), vinca alkaloids (vinblastine and vincristine) and cyclosporine. ⁵ In affected cats, substrate macrocyclic lactones, such as eprinomectin, accumulate within the central nervous system and bind to the gamma-aminobutyric acid type A (GABA_A)-gated chloride channel receptors, resulting in mydriasis, tremors, ataxia, paresis and, in some cases, death.^1,3 Drugs are eventually cleared from the central nervous system through diffusion. This patient demonstrated clinical signs consistent with macrocyclic lactone toxicity after application of an eprinomectin-containing product and was subsequently identified to be homozygous for ABCB11930_1931del TC, the first documented case in Australia.

Case description

A 9-month-old castrated male Maine Coon presented for evaluation at a primary care clinician in Sydney, Australia, for tongue protrusion and lethargy 24 h after administration of NexGard Spectra (Boehringer Ingelheim; active ingredients praziquantel 83 g/l, esafoxolaner 12 g/l, eprinomectin 4 g/l). The cat had been previously treated with this product, developing transient tongue protrusion lasting for up to 2 days. Another cat in the household, a genetically unrelated Maine Coon, received the same product simultaneously and was clinically well. The patient was an indoor-only cat fed a commercial balanced diet with no other prior pertinent medical history.

On initial examination (day 1 after application), the cat was of a healthy weight (5.6 kg), with a body condition score of 4/9. Vital signs were normal. The cat demonstrated tongue protrusion with pelvic limb ataxia. A menace response was absent bilaterally, with other cranial nerve reflexes remaining intact. The cat was treated with pregabalin 3.6 mg/kg PO once, monitored in hospital and subsequently discharged into the owner’s care that afternoon.

The following day, the cat developed dysphagia and re-presented to the primary care veterinarian. Vital signs were within reference intervals (RIs). The patient was dull with ongoing tongue protrusion, minimal gag and bilateral mydriasis. Although ambulatory, the patient was ataxic with intermittent muscle twitching. Haematology and biochemistry were submitted to an external reference laboratory, and the patient was transferred to an overnight emergency care provider.

The EDTA sample clotted, precluding a complete blood count. Serum biochemistry indicated a mild increase in alanine transaminase (ALT) at 110 U/l (RI 19–100) and aspartate aminotransferase at 68 U/l (RI 2–62) with a normal creatine kinase of 132 U/l (RI 0–314) and marginal hyperphosphataemia of 2.5 mmol/l (RI 1.0–2.3). An apparent hypoglycaemia of 2.1 mmol/l (RI 3.2–7.5) was documented from a sample obtained from a plain tube.

Upon presentation to the emergency care provider, the patient was quiet with bilateral mydriasis (Figure 1). The patient was tachypnoeic (52 breaths/min, normal effort) with a normal heart rate (180 beats/min) and rectal temperature (38.3°C). Tongue protrusion, delayed menace response and a weak gag persisted. Other cranial nerve reflexes were assessed as intact. The patient was generally weak but could walk with assistance. After discussion with the Australian Animal Poisons Centre, no specific treatment was recommended given the time from initial application (>48 h). Serum glucose was repeated and was normal at 5.3 mmol/l (RI 3.5–7.5). The patient was placed on intravenous fluid therapy (lactated Ringer’s solution, 11 ml/h IV).

Figure 1

The patient on day 3, demonstrating bilateral mydriasis and tongue protrusion

The following morning (day 3), the patient was quiet but responsive with a normal respiratory rate (20–24 breaths/min). Mydriasis, delayed menace and generalised weakness persisted. The pupillary light reflexes were slightly delayed bilaterally. Physiological nystagmus was sluggish with absent spontaneous nystagmus or strabismus. Facial sensation appeared generally reduced and there was no sign of sensation with palpation of the rostral aspect of the tongue. Distal tongue sensation was present, as was a detectable and repeatable strong gag reflex. The patient was ambulatory but weak. Paw placement and proprioception were normal. Patellar reflexes were present but subjectively delayed, presumably due to generalised weakness.

Haematology was repeated, given the previously clotted sample, which was normal. An in-house ammonia test was normal at 10 umol/l (RI 0–95), as was a repeat serum glucose at 5.2 mmol/l (RI 3.5–7.5). A bedside cryptococcal lateral flow antigen assay (CrAg LFA; IMMY) was negative. Regrettably, a repeat ALT was not performed. Bedside retroviral testing (Witness FeLV-FIV; Zoetis) was negative. Further diagnostics, including an MRI of the brain and cerebrospinal fluid analysis, were discussed but declined. Multifocal central nervous system pathology was suspected, with the clinical history and examination findings consistent with macrocyclic lactone neurotoxicity.

Throughout the day, the cat demonstrated an ability to safely prehend and swallow food and water and was able to urinate and defecate without assistance. Given that no further testing was being undertaken, the patient was considered stable for discharge. Communications at 2 days, 7 days, 1 month and 3 months after admission indicated gradual resolution of all neurological signs. The cat was neurologically normal 3 months from initial application, with tongue protrusion being the last clinical sign to resolve.

A temporal relationship between the application of the topical anti-parasiticide and neurological signs was interrogated. NexGard Spectra contains the active ingredients praziquantel 83 g/l, esafoxolaner 12 g/l and eprinomectin 4 g/l, with eprinomectin serving as a substrate for the drug transporter molecule P-gp. ⁵ Sparse reports in the literature have identified cats with similar neurological signs after application of macrocyclic lactone-containing products at label doses,^2,3,6 with genetic sequencing identifying many to be homozygous for a genetic variant ABCB11930_1931del TC, which codes for this molecule. ⁷

A buccal swab was submitted for genome sequencing of ABCB11930_1931del TC through the Program in Individualized Medicine at the College of Veterinary Medicine, Washington State University, USA. The patient was homozygous for this variant, inferring an inherent deficiency in P-gp and thus likely neurotoxicosis from administration of macrocyclic lactone. The other cat in the household was homozygous for the wild-type allele, indicating a functional P-gp and ability to safely handle substrate drugs.

Discussion

This is the first reported cat in Australia to be homozygous for ABCB11930_1931del TC with an adverse drug reaction to a registered, commercially available parasiticide at label doses.

Neurotoxicosis has been documented in cats after administration of topical, subcutaneous and oral anti-parasiticides containing macrocyclic lactones. ¹ Genomic sequencing of cats with neurotoxocosis after administration of ivermectin and selamectin at standard doses led to the identification of a nonsense mutation in ABCB11930_1931del TC. Cats homozygous for this variant were found to develop a 2 base pair deletion at exon 15, creating a series of stop codons downstream from the deletion resulting in a truncated P-gp, rending this protein non-functional.^3,4,8 The frequency of this variant in the general feline population appears to be low, with estimates in the range of 0.6–4% for cat breeds overall, and up to 5.4% in Maine Coons.^3,6,9 The frequency in Australia is unknown.

Eprinomectin-containing products have only recently been approved by veterinary regulatory bodies for parasite prevention in cats. The most widely available product, manufactured by Boehringer Ingelheim Animal Health Australia, and used in this case, underwent required safety studies for release into the veterinary market. When the product containing praziquantel 83 g/l, esafoxolaner 12 g/l and eprinomectin 4 g/l was applied to the skin of 21 colony cats at one, three and five times the label dose, or when administrated orally at the standard dose, no cat demonstrated clinical signs of neurotoxicosis.^10,11

When eprinomectin-containing products became commercially available, the Program in Individualized Medicine at the College of Veterinary Medicine, Washington State University, USA, identified a temporal cluster in cats with neurotoxicosis after application of an eprinomectin-containing product (NexGard Combo, Boehringer Ingelheim; active ingredients praziquantel 83 mg/ml, esafoxolaner 12 mg/ml, eprinomectin 4 mg/ml). ³ Of 26 samples submitted for sequencing with clinical signs after application, 14 were homozygous for ABCB11930_1931del TC. ² As eprinomectin is a P-gp substrate, a causal relationship between cats homozygous for ABCB119310_1931del TC and signs of neurotoxicosis after application was likely. Like dogs with the ABCB1-1 Delta variant, a deficiency in this protein leads to the accumulation of eprinomectin within the central nervous system and binding to GABA_A-gated chloride channels.^1,5

Treatment in this case was lately supportive, given the general clinical improvement over the 24 h at the secondary veterinary institution. As a result of the lag from application to development of clinical signs, surface decontamination through topical washing would likely have had minimal clinical effect. However, it could be considered as a first-line treatment option for cats with intrinsic P-gp deficiency when performed within the first few hours of application. Intra-lipid emulsion therapy, a potent xenobiotic that binds to avermectin and promotes plasma clearance, has been described in both dogs and cats with macrocyclic lactone toxicity. ¹ Although there is no clear link between administration and improved clinical outcomes, several authors support the use for cats with severe clinical presentation, including stupor, coma and seizures. Should this cat have clinically progressed, this would have been a reasonable treatment consideration.

The patient in this case study made an uneventful recovery over a 4-week period, with tongue protrusion being the most persistent clinical sign. One could argue that this cat may have experienced a toxic hepatopathy, given the ALT on day 2 was marginally increased at 110 U/l (RI 19–100). As the ammonia was normal on day 3 and the patient was improving, hepatic encephalopathy was considered highly unlikely. Subsequent evaluation at 1 week from presentation was normal at ALT 80 U/l (RI 19–100). Although poorly specific, abdominal ultrasonography could have been utilised to identify sonographic changes of the liver during initial hospitalisation that could have been associated with the presenting clinical signs.

The exact reason as to why this patient developed severe neurotoxicosis with this specific application of NexGard Spectra is unknown. Cats are fastidious groomers, and it is possible that the patient ingested a quantity of his own and/or the other cat’s medication after application. Although safety studies indicated that the product was safe with oral ingestion, a cat homozygous for ABCB11930_1931del TC, as in this case, is likely to have altered elimination of the product due to a P-gp deficiency and subsequent accumulation within the central nervous system.

The Program in Individualized Medicine at Washington State University, USA currently offers an MDR1 test to detect ABCB11930_1931del TC in cats. Cats who develop adverse clinical signs after application of an eprinomectin or P-gp substrate drug, or any relative of a cat with confirmed homozygosity for ABCB11931_1930del TC, should be assessed through this or other commercially available testing programmes.

Conclusions

This is the first cat in Australia with signs of macrocyclic lactone toxicity after administration of a product containing eprinomectin at label doses known to be homozygous for ABCB11930_1931del TC. Further awareness of this variant and knowledge of substrate drugs is required to improve the safety of these products in the general cat population and to help improve patient care.