主要用來評估藥品生體相等性的方法包括藥動學、藥效學、臨床試驗及體外釋出試驗。傳統的藥動學是以血液中的藥品濃度作為評估，但外用皮膚製劑主要是針對局部使用且全身吸收甚少。以藥效學為評估方法時則難以建立藥效反應的評估模式。若用臨床試驗作為其評估生體相等性的方法，所需時間較長且成本較高。對於外用皮膚製劑的體外試驗是以體外釋出試驗進行，即測定藥品通過一合成膜的釋出速率與總量，但有時與體內試驗的結果無法完全相符。故美國AAPS/FDA建議可採用皮膚藥動試驗法。皮膚藥動試驗法(Dermatopharmatokinetics / DPK)，是建立在皮膚角質層為藥物經皮吸收的主要速率限制步驟，經由皮膚層撕法（Skin stripping technique）和藥物活性成分之分析，可對角質層上的活性成分藥量之動態變化作評估，此外亦可利用皮膚層撕的技術評估藥品在角質層上吸收達飽和時的藥量。
　　Tretinoin (all-trans retinoic acid, Vitamin A acid) 即維他命A酸，是臨床上常用於治療青春痘、粉刺的外用藥品之一。本研究的目的為利用體外釋出試驗、單一時間點皮膚層撕評估外用Tretinoin製劑不同濃度、製劑、廠牌的效應，利用皮膚藥動學試驗法評估兩0.05% Tretinoin不同廠牌乳膏的生體相等性，及比較此三種試驗的關聯性。選用的Tretinoin製劑有0.01%、0.025% Retin-A凝膠，0.025%、0.05% Retin-A乳膏、0.05% Airol乳膏。
　　Tretinoin製劑經由Silicon membrane的體外釋出試驗研究結果顯示，當所給予之同劑型產品濃度增加，藥品的釋出速率和總量也會隨之增加。0.025% Retin-A凝膠的的釋放速率和六小時累積滲出量是0.01% Retin-A凝膠的3.9和3.4倍，0.05% Retin-A乳膏的釋放速率和六小時累積滲出量是0.025% Retin-A乳膏的1.5和1.3倍，且凝膠的釋放速率較同濃度乳膏快，而Airol的釋放速率相對於同濃度的Retin-A只有十分之ㄧ。在PTFE membrane的體外釋出試驗結果，發現Retin-A凝膠無法經由此種膜釋出，但Retin-A乳膏在兩種膜的釋出速率和總量相似，而Airol乳膏的結果則近似於同濃度的Retin-A乳膏。
　　給予不同濃度、廠牌的Tretinoin製劑對於以皮膚層撕技術撕取下的角質層量沒有顯著影響，但給予乳膏後撕取下的角質層量會較給予凝膠的部位多，且凝膠和乳膏在給藥2小時後皆可達吸收飽和的狀態。给藥2小時後，在給予0.025% Retin-A凝膠的部位角質層上的藥量是0.01% Retin-A凝膠的2.2倍，給予0.05% Retin-A乳膏的部位角質層上的藥量是0.025% Retin-A乳膏的1.3倍，且凝膠劑型在角質層上測得的Tretinoin量較同濃度乳膏多，而給予Airol後在角質層上測得的藥量相對於同濃度的Retin-A約有1.5倍之多。
　　依據皮膚藥動學試驗法的研究結果顯示，0.05% Airol乳膏Tretinoin和Total retinoids的Amax及AUC0-14約為0.05% Retin-A乳膏的1.5倍，且統計上具有顯著差異，與給藥2小時皮膚層撕的結果相似，然而體外經PTFE membrane的釋出試驗則無法辨別此差異，故皮膚藥動學試驗法的鑑別度較高，應可用於評估外用Tretinoin製劑的生體相等性。

　　Bioequivalence assessment can be evaluated by 4 methods in order of preference: pharmacokinetic, pharmacodynamic, comparative clinical trials, or in vitro studies. Traditional pharmacokinetics evaluates the active drug concentration in the blood. Dermatologic drug products, however, are designed to target the local tissue have limited systemic absorption. Using pharmacodynamic response to determine bioequivalence is usually hard to estabilish the evaluated principle. Clinical trials are generally time consuming and expensive. In vitro release test is the method of in vitro study for topical dermatological products. Drug release rate and extent from topical dermatological products can be measured using a synthetic membrane. In general, to obtain in vitro - in vivo correlation is difficult. Dermatophatmacokinetics / DPK is based on the assumption that the stratum cornum (SC) is the rate-limiting step for topical drug absorption. DPK studies provide information on drug concentration in the SC with respect to time and should include validation of both analytical methods and the technique of skin stripping.
　　Tretinoin is the structural and functional analogus of vitamine A. Topical tretinoin products are used to treat acne. The purpose of this study was to evaluate the effect of concentration, formulation and manufacturer on the performance of topical tretinoin products by in vitro release and skin stripping studies, to assess bioequivalence of two tretinoin creams from different manufacturers by dermatopharmacokinetic (DPK) studies, and comparison of in vitro release, skin stripping, and DPK of topical tretinoin products. The tretinoin products used included 0.01% and 0.025% Retin-A gel, 0.025% and 0.05% Retin-A cream, and 0.05% Airol cream.
　　The results of in vitro release across silicon membrane revealed that increasing tretinoin concentration in the products resulted in an increasing rate and extent of drug released. Increasing tretinoin concentration from the 0.025% to 0.1% in Retin-A gel produced a 3.9-fold increase in the rate of drug released and a 3.4-fold increase in the extent of drug released. Increasing drug concentration in the Retin-A creams from 0.025% to 0.5% produced a 1.5-fold increase in the rate of drug released and a 1.3-fold increase in the extent of drug released. The release rates of gel formulation were higher than these of cream formulation at the same strength. The rate and extent of drug released with 0.05% Retin-A cream was 10-fold of Airol cream. No released from Retin-A gel was detected across PTFE membrane. In contrast, the rate and extent of both Retin-A creams across silicon and PTFE membrane were similar. The release rate of Airol cream across PTFE membrane was similar with 0.05% Retin-A cream.
　　SC weight harvested from cream-treated sites was greater than that from gel-treated sites. However, drug contents measured in the harvested SC were greater from gel formulations than those from creams. SC uptake of tretinoin from both gel and cream applications have reached the saturated level within 2 hours. After 2 hours of product applications the SC drug content achieved with 0.025% Retin-A gel was 2.2-fold greater than the drug content produced with 0.01% gel and 0.05% Retin-A cream was 1.3-fold greater than 0.025% cream. The drug amount in the SC with gel-treated sites was greater than cream-treated sites at the same strength. Airol cream produced 1.5-fold greater drug contents in the harvested SC than Retin-A cream.
　　The DPK study demonstrated that the Amax and AUC0-14 of both tretinoin and total tretinoin from 0.05% Airol cream were 1.5-fold significantly greater than these from 0.05% Retin-A cream. This result was consistent with skin stripping studies after 2 hours of product applications, but disagreed with the in vitro release studies. These results suggested DPK method is more discriminative and maybe appropriate for bioequivalence assessment of topical treitnoin products.

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