N-acetylcysteine is a thiol-containing antioxidant, which has shown otoprotective effects in in-vitro as well as in-vivo models of cisplatin induced hearing loss. Systemic administration of antioxidants, however, is associated with the major potential drawback of interference with the tumoricidal effect of cisplatin. This therapeutic limitation can be overcome by local intratympanic injection of the antioxidant N-acetylcysteine, which results in very restricted systemic uptake of the drug, whilst intracochlear drug levels are substantially higher. Furthermore, osmolality and pH properties of formulations for intratympanic injection need to be controlled, as they impact the fraction of drug crossing the barriers of the inner ear and could potentially damage middle and inner ear structures. This study focused on (i) the evaluation of concentration-time profiles of N-acetylcysteine in perilymph, cerebrospinal fluid and plasma after intratympanic administration, (ii) the influence of the dosage form, i.e. a thermoreversible poloxamer 407 hydrogel versus a solution, on N-acetylcysteine pharmacokinetics, and (iii) the development of a pH- and osmolality-adjusted formulation for intratympanic N-acetylcysteine delivery.

49 female albino guinea pigs were randomized into two treatment groups, receiving either a single intratympanic injection of a 4% N-acetylcysteine poloxamer 407 hydrogel or a 4% N-acetylcysteine solution. 8 animals served as untreated controls. N-acetylcysteine levels in perilymph, cerebrospinal fluid and plasma were monitored over a period of 24 h. Samples were taken at 1, 3, 6, 12 and 24 h (poloxamer 407 hydrogel group) and 1, 6 and 24 h (solution group) post injection, and analysed by high performance liquid chromatography-tandem mass spectrometry. Intratympanic application of the 4% N-acetylcysteine poloxamer 407 hydrogel resulted in a 4-fold larger perilymph area under the concentration-time curve (0-24 h) than topical administration of the equally concentrated N-acetylcysteine solution but in similar plasma N-acetylcysteine levels. N-acetylcysteine concentrations in the cerebrospinal fluid were below the level of detection (5 ng/ml) in both treatment groups. N-acetylcysteine-containing formulations applied to the middle ear were isohydric and osmolality was reduced by up to 200 mosmol/kg compared to equally concentrated formulations used in previous studies.

In summary, we were able to demonstrate that the intratympanic injection of thermoreversible poloxamer 407 hydrogels increases and sustains N-acetylcysteine delivery to the inner ear. Given the low plasma N-acetylcysteine levels after topical application and the physiological pH and osmolality of the hydrogel, the risk of compromising the antineoplastic effects of cisplatin therapy and of local side effects is minimal.

N-乙酰半胱氨酸是一种含硫醇的抗氧化剂，在顺铂诱导的听力损失的体外和体内模型中均显示了耳保护作用。然而，抗氧化剂的全身给药与干扰顺铂的杀肿瘤作用的主要潜在缺点有关。通过局部鼓膜内注射抗氧化剂N-乙酰半胱氨酸可以克服这种治疗局限性，这会导致药物的全身吸收受到极大限制，而耳蜗内的药物水平则要高得多。此外，需要控制用于鼓膜内注射的制剂的重量克分子渗透压浓度和pH值特性，因为它们会影响穿过内耳屏障的药物比例，并可能损坏中耳和内耳结构。

将49只雌性白化豚鼠随机分为两个治疗组，分别接受鼓膜内注射4％N-乙酰半胱氨酸泊洛沙姆407水凝胶或4％N-乙酰半胱氨酸溶液。8只动物作为未处理的对照。在24小时内监测外周淋巴液，脑脊液和血浆中的N-乙酰半胱氨酸水平。注射后第1、3、6、12和24小时（泊洛沙姆407水凝胶组）和1、6和24小时（溶液组）取样，并通过高效液相色谱-串联质谱分析。鼓膜内应用4％N-乙酰半胱氨酸泊洛沙姆407水凝胶在浓度-时间曲线（0-24小时）下的局部淋巴面积比局部给药相同浓度的N-乙酰半胱氨酸溶液但在相似的血浆N-中大4倍。乙酰半胱氨酸水平。在两个治疗组中，脑脊液中的N-乙酰半胱氨酸浓度均低于检测水平（5 ng / ml）。与先前研究中使用的等浓度制剂相比，应用于中耳的含N-乙酰半胱氨酸的制剂具有等渗性，重量摩尔渗透压浓度降低了200 mosmol / kg。

总之，我们能够证明鼓膜内注射热可逆的泊洛沙姆407水凝胶会增加并维持N-乙酰半胱氨酸向内耳的传递。鉴于局部应用后血浆N-乙酰半胱氨酸水平较低以及水凝胶的生理pH和重量摩尔渗透压浓度，损害顺铂治疗的抗肿瘤作用和局部副作用的风险极小。