This study aimed to investigate both the pharmacokinetic behavior and tolerance of methotrexate (MTX) in horses to design a specific dosing regimen as a new immunomodulatory drug for long‐term treatment. To determine the primary plasma pharmacokinetic variables after single intravenous, subcutaneous or oral administration, six horses were administered 0.3 mg/kg MTX in a crossover design study. After a 10‐week washout, MTX was administered subcutaneously to three of the six previously treated horses at a dose of 0.3 mg/kg once per week for 3 months. In both studies, MTX and metabolite concentrations were measured using LC‐MS/MS. The absolute bioavailability of MTX was 73% following subcutaneous administration but less than 1% following oral administration. The plasma clearance was 1.54 ml min⁻¹ kg⁻¹ (extraction ratio = 2%). After 24 hr, plasma concentrations were below the LOQ. No adverse effects were noted except for a moderate reversible elevation in liver enzymes (GLDH). With regards to the main metabolites of MTX, very low concentrations of 7‐hydroxy‐MTX were found, whereas polyglutamated forms (mainly short chains) were found in red blood cells. A subcutaneous dose of 0.2 mg kg⁻¹ week⁻¹ may be safe and relevant in horses, although this has yet to be clinically confirmed.

中文翻译：

---

小剂量甲氨蝶呤在马中的药代动力学。

这项研究旨在研究甲氨蝶呤（MTX）在马中的药代动力学行为和耐受性，以设计特定的给药方案作为长期治疗的新免疫调节药物。为了确定单次静脉内，皮下或口服给药后主要的血浆药代动力学变量，在交叉设计研究中给六匹马给药0.3 mg / kg MTX。经过10周的冲洗后，以0.3 mg / kg的剂量皮下给予6匹先前治疗的马匹中的3匹，每周一次，持续3个月。在两项研究中，均使用LC-MS / MS测量了MTX和代谢产物的浓度。皮下给药后MTX的绝对生物利用度为73％，但口服后小于1％。血浆清除率为1.54 ml min ^-1  kg^-1（提取率= 2％）。24小时后，血浆浓度低于LOQ。除了肝酶（GLDH）有中等程度的可逆升高外，未观察到不良反应。关于MTX的主要代谢产物，发现了极低浓度的7-羟基-MTX，而在红细胞中发现了多谷氨酸形式（主要是短链）。尽管尚未得到临床证实，皮下注射0.2 mg kg ^-1 周^-1的剂量在马中可能是安全的并具有相关性。