Synthesis And Characterization Of The Potential Prodrugs For The CNS Delivery Of Tacrine

Alzheimer's disease (AD) is a kind of chronic and progressive neurodegenerative disorder with the organic pathological lesion of the brain, which commonly happens in the senility or presenility. It is extensively accepted that AD is one of the severe diseases that threaten the health and affect the living quality of the old people. Owing to the presence of the blood brain barrier, most of the drug cannot or little were delivered into the brain following systemic administration, which make it difficult for the treatment of AD. Tacrine hydrochloride (THA) is the first acetylcholinesterase inhibitor approved by FDA for palliative treatment of mild and moderate AD. Despite clinical evidence of beneficial activity against AD, tacrine is not an ideal drug because of its short half-life, narrow therapeutic window, and severe hepatotoxicity. Now it has been removed from the clinic.According to the lipid characteristics of the BBB, the lipophilicity of THA was enhanced by the way of the prodrug to overcome the limited access to the brain and to explore the possibility of increasing the curative effect and decreasing the side effect. The prodrugs of THA were synthesized firstly. Then their related physiochemical properties and stabilities were investigated. The feasibility of the prodrugs transporting through the BBB was assessed bychromatographic model in vitro. The tissue distribution and pharmacokinetics -of PTHA were also studied in mice and compared with that of THA. Furthermore, the acute toxicity of PTHA was determined.iV-benzoyl amide-THA(BTHA), iV-dibenzoyl amide-THA(DBTHA), N-probenecid-THA(PTHA) and iV-linoleic acidamide-THA(LTHA) were synthesized using THA, benzoyl chloride, probenecid and linoleic acid as the raw materials. Their structures were confirmed by mass spectrometry, UV spectrophotometry, IR spectrophotometry and *H NMR studies.The stabilities of the prodrugs were investigated in the buffer with various pH values, at various temperatures and in mice plasma in vitro. The stability of DBTHA was affected by pH and temperature. DBTHA was easy to be hydrolyzed in the basic or acidic environment while it was relatively stable in the neutral condition. Hydrolysis was accelerated when the temperature increased. DBTHA also showed its unstability under the action of some enzymes in mice plasma. But the other prodrugs were stable in the various experimental conditions.The prodrugs are more lipophilic than tacrine by determining the solubility and the partition coefficient, which indicates that lipophilicity of the parent drug was increased by chemical modification. Assayed by the biopartition micellar chromatography, the retention time of THA, BTHA, DBTHA and PTHA were 11.4, 41.5, 88.6 and 104.6 min, respectively. Therefore, the permeability of the prodrugs may be predicted as follows: LTHA > PTHA > DBTHA >BTHA> THA.The HPLC method was set up to determine the concentrations of PTHA in the tissues including heart, liver, spleen, lung, kidney, brain and plasma of themice. The recovery and precision could satisfy the determination requirements about the biosamples.The tissue distribution and pharmacokinetics of PTHA and THA were determined by HPLC methods after i.v. administration of PTHA or THA into mice, respectively. The results indicated that compared with direct injection of THA, the biodistribution and pharmacokinetics of THA altered markedly after injection of PTHA. The beginning concentration of PTHA was higher in the brain and eliminated very quickly. But THA concentration released from PTHA always kept stable at about 0.5 ug/g in the brain and about 0.2 ug/g in the liver. The activation time was prolonged from 2h to 4h. Although AUC of the released THA decreased from 2.90ug-h/g to 2.35jig-h/g, TE increased from 10.81% to 14.61% in the brain. At the same time, the distribution volume of THA decreased from 1.91ug-h/g to 0.93ug*h/g and TE decreased from 1.91ug-h/g to 0.93ug-h/g in the liver. Accordingly, AUCbrain/AUQiver increased from 1.52 to 2.53. These alterations demonstrated that PTHA had the potential to improve curative effects and decrease the hepatotoxicity of THA.The LD50 of PTHA was determined to be 55.56mg/kg, which was 6.14 times higher than that of THA (3.85mg/kg). Apparently, PTHA had much lower toxicity than that of THA.These results suggest that PTHA have attractive features to increase the therapeutic effect and decrease the adverse reaction of THA.