Keiko Yamashita

Comparative studies of polyethylene glycol-modified liposomes prepared using different PEG-modification methods.

To address the issue of excess polyethylene glycol (PEG)-lipid degradation observed when PEG-modified liposomes are prepared using the pH-gradient method, a concept using a novel PEG-modification method, called the post-modification method, was proposed and evaluated. To assess the proof concept, a preservation-stability study and a pharmacokinetic study were performed that compared the conventional PEG-modification method, called the pre-modification method, with the post-modification method. The results show that PEG-lipid degradation could be markedly inhibited in the post-modification method. Furthermore, the post-modification method could be used without any manufacturing process difficulties, especially with high PEG-lipid content. In addition, a higher blood circulation capability was observed in the post-modification method. Through comparative studies, it was found that the post-modification method was advantageous compared to the pre-modification method. In conclusion, the post-modification method has the potential to be a novel PEG-modification method that can achieve a higher preservation stability of PEG-lipid, a greater ease of manufacturing, and a higher blood circulation capability, especially in the manufacturing of pH-gradient liposomal products.

Comparative studies of irinotecan-loaded polyethylene glycol-modified liposomes prepared using different PEG-modification methods.

Recently, a polyethylene glycol (PEG)-modification method for liposomes prepared using pH-gradient method has been proposed. The differences in the pharmacokinetics and the impact on the antitumor effect were examined; however the impact of PEG-lipid molar weight has not been investigated yet. The main purpose of this study is to evaluate the impact of PEG-lipid molar weight against the differences in the pharmacokinetics, the drug-release profile, and the antitumor effect between the proposed PEG-modification method, called the post-modification method, and the conventional PEG-modification method, called the pre-modification method. Various comparative studies were performed using irinotecan as a general model drug. The results showed that PEG-lipid degradation could be markedly inhibited in the post-modification method. Furthermore, prolonged circulation time was observed in the post-modification method. The sustained drug-release was observed in the post-modification method by the results of the drug-releasing test in plasma. Moreover, a higher antitumor effect was observed in the post-modification method. It was also confirmed that the same behaviors were observed in all comparative studies even though the PEG molecular weight was lower. In conclusion, the post-modification method has the potential to be a valuable PEG-modification method that can achieve higher preservation stability of PEG-lipid, prolonged circulation time, and higher antitumor effect with only half the amount of PEG-lipid as compared to the pre-modification method. Furthermore, it was demonstrated that PEG(5000)-lipid would be more desirable than PEG(2000)-lipid since it requires much smaller amount of PEG-lipid to demonstrate the same performances.

Designing a novel in vitro drug-release-testing method for liposomes prepared by pH-gradient method.

In order to evaluate the drug-release behavior from pH-gradient liposomal formulation, a simple release-testing method without using biological components was newly designed on the basis of inversed ammonia gradient principle. Various factors influencing drug-release (releasing factor) were examined. As a result, releasing factors concentration, pH, osmolarity in test fluid, and releasing factors structure were found to be the critical factors to be optimized. Various vincristine-loaded liposomes with different lipid compositions or with different lipid/cholesterol ratio were tested for drug-release behavior and successfully obtained drug-release profiles reflecting differences in the physicochemical properties of individual liposomes. Furthermore, since the comparative release study of vincristine-loaded liposomes and doxorubicin-loaded liposomes could reproduce the phenomena as other researchers recently reported, a possibility was suggested for the proposed method to estimate the physicochemical status of drug inside of liposomes. Proof of concept study concluded, as a whole, that the novel release-testing method would be useful for a formulation study and also useful as a tool for the quality assurance or quality control in the manufacturing of pH-gradient liposomal products.