Plasma Protein Binding of Amphotericin B and Pharmacokinetics of Bound versus Unbound Amphotericin B after Administration of Intravenous Liposomal Amphotericin B (AmBisome) and Amphotericin B Deoxycholate

Unilamellar liposomal amphotericin B (AmBisome) (liposomal AMB) reduces the toxicity of this antifungal drug. The unique composition of liposomal AMB stabilizes the liposomes, producing higher sustained drug levels in plasma and reducing renal and hepatic excretion. When liposomes release their drug payload, unbound, protein-bound, and liposomal drug pools may exist simultaneously in the body. To determine the amounts of drug in these pools, we developed a procedure to measure unbound AMB in human plasma by ultrafiltration and then used it to characterize AMB binding in vitro and to assess the pharmacokinetics of nonliposomal pools of AMB in a phase IV study of liposomal AMB and AMB deoxycholate in healthy subjects. We confirmed that AMB is highly bound (>95%) in human plasma and showed that both human serum albumin and α(1)-acid glycoprotein contribute to this binding. AMB binding exhibited an unusual concentration dependence in plasma: the percentage of bound drug increased as the AMB concentration increased. This was attributed to the low solubility of AMB in plasma, which limits the unbound drug concentration to <1 μg/ml. Subjects given 2 mg of liposomal AMB/kg of body weight had lower exposures (as measured by the maximum concentration of drug in serum and the area under the concentration-time curve) to both unbound and nonliposomal drug than those receiving 0.6 mg of AMB deoxycholate/kg. Most of the AMB in plasma remained liposome associated (97% at 4 h, 55% at 168 h) after liposomal AMB administration, so that unbound drug concentrations remained at <25 ng/ml in all liposomal AMB-treated subjects. Although liposomal AMB markedly reduces the total urinary and fecal recoveries of AMB, urinary and fecal clearances based on unbound AMB were similar (94 to 121 ml h(−1) kg(−1)) for both formulations. Unbound drug urinary clearances were equal to the glomerular filtration rate, and tubular transit rates were <16% of the urinary excretion rate, suggesting that net filtration of unbound drug, with little secretion or reabsorption, is the mechanism of renal clearance for both conventional and liposomal AMB in humans. Unbound drug fecal clearances were also similar for the two formulations. Thus, liposomal AMB increases total AMB concentrations while decreasing unbound AMB concentrations in plasma as a result of sequestration of the drug in long-circulating liposomes.

单层脂质体两性霉素B（AmBisome，脂质体AMB）可降低该抗真菌药物的毒性。脂质体AMB独特的组分可稳定脂质体结构，使血浆中药物浓度维持在较高水平，并减少经肾脏与肝脏的排泄量。当脂质体释放所载药物时，体内可同时存在游离型、蛋白结合型及脂质体结合型三类药物池。为定量测定此类药物池中的药物含量，我们开发了一种通过超滤法检测人血浆中游离两性霉素B的实验方法，并将其用于两项研究：一是体外表征两性霉素B的蛋白结合特性，二是针对健康受试者开展的脂质体AMB与脱氧胆酸两性霉素B的IV期临床试验，以评估非脂质体结合型两性霉素B的药代动力学行为。我们证实，两性霉素B在人血浆中具有极高的蛋白结合率（＞95%），且人血清白蛋白与α1-酸性糖蛋白均参与了该结合过程。两性霉素B的血浆结合特性呈现出异常的浓度依赖性：结合型药物的占比随两性霉素B浓度升高而上升，这一现象可归因于两性霉素B在血浆中的低溶解度，其游离药物浓度被限制在＜1 μg/ml的范围内。与接受0.6 mg/kg体重脱氧胆酸两性霉素B的受试者相比，给予2 mg/kg体重脂质体AMB的受试者，其游离型与非脂质体结合型药物的暴露量（以血清药物峰浓度及浓度-时间曲线下面积衡量）更低。在给予脂质体AMB后，血浆中绝大多数两性霉素B仍与脂质体结合（给药4小时时为97%，168小时时为55%），因此所有接受脂质体AMB治疗的受试者体内游离药物浓度均维持在＜25 ng/ml的水平。尽管脂质体AMB显著降低了两性霉素B的总尿液与粪便回收率，但基于游离两性霉素B计算的尿液与粪便清除率在两种制剂中相近（94~121 ml·h⁻¹·kg⁻¹）。游离药物的尿液清除率等同于肾小球滤过率，肾小管转运速率仅为尿液排泄速率的＜16%，这提示常规剂型与脂质体型两性霉素B在人体中的肾脏清除机制均为游离药物的净滤过，几乎无分泌或重吸收过程。两种制剂的游离药物粪便清除率亦相近。综上，脂质体AMB通过将药物包封于长循环脂质体中，提升了血浆中总两性霉素B浓度，同时降低了游离药物浓度。