一些核苷类化合物活性不高的主要原因是其不能有效地被核苷激酶磷酸化或者根本就不是核苷激酶的底物；另一方面，磷酸化的核苷（核苷酸）分子本身通常由于稳定性差、极性大不易透过细胞膜等因素而难以成药。为了能够绕过单磷酸化的限速步骤，同时改善化合物的稳定性和脂溶性，可将相应的核苷酸（核苷单磷酸）修饰成其前体药物。该策略可以直接、有效地将核苷酸输入到细胞内，从而大大提高药物的活性，降低给药剂量和毒副作用。本项目拟基于本课题组原创的苄氧磷酰胺肝靶向前药技术，将前期发现的具有抗乙肝病毒活性的2'-脱氧-2'-氟-4'-叠氮核苷分子合成核苷酸类前药。通过前药基团优化、体外活性评价，以及体内药效学研究等，以获得具有肝靶向性、高效低毒，且可以口服的新型核苷酸前药类的候选药物用于慢性乙肝的治疗。

Some nucleosides are weakly active because they cannot be efficiently phosphorylated by specific nucleoside kinases or are not substrates of the kinases at all. On the other hand, nucleoside phosphates (nucleotides) per se cannot be used as drugs very often because they are chemically less stable and/or too polar to enter the cells. In order to bypass the rate-limiting first-step phosphorylation, and meanwhile to improve the stability and lipophilicity of compound, the corresponding nucleotides (nucleoside monophosphates) could be modified into their prodrugs. Such strategies could deliver the nucleoside phosphates into the cells directly and efficiently, therefore significantly improve the biological activity, and reduce the drug dose and side effects. In this project, employing our liver-targeted phosphoramidate prodrug strategy, we plan to design and synthesize a series of nucleotide pordrugs of anti-HBV 2'-deoxy-2'-fluoro-4'-azido nucleosides that we discovered previously. Through optimization of the prodrug substructure, in vitro evaluation, and in vivo efficacy study, novel liver-targeted and orally-active prodrug candidates with high activity and low toxicity will be identified for the treatment of chronic hepatitis B.