Amino Acid Promoieties Alter Valproic Acid Pharmacokinetics and Enable Extended Brain Exposure.

Valproic acid (VPA) has been used to treat epileptic seizures for decades, but it may also possess therapeutic potential in other nervous system diseases. However, VPA is extensively bound to plasma proteins, asymmetrically transported across the blood-brain barrier and metabolized to toxic species in the liver, which all contribute to its severe off-target adverse effects and possible drug-drug interactions. In this study, we evaluated seven amino acid prodrugs of VPA that were targeted to utilize L-type amino acid transporter 1 (LAT1), if they could alter the brain uptake mechanism and systemic pharmacokinetics of VPA. All prodrugs had affinity for LAT1 studied as competitive inhibition of [14C]-L-leucine in human breast cancer (MCF-7) cell line. However, since the ester prodrugs were unstable they were not studied further, instead the corresponding amide prodrugs were used to evaluate their systemic pharmacokinetics in rats and the uptake mechanism via LAT1 into the rat brain. All amide prodrugs were bound to a lesser extent to plasma proteins than VPA and this being independent of the prodrug concentration. Amide prodrugs were also delivered into the brain after intravenous bolus injection. One of the prodrug showed greater brain uptake and high selectivity for LAT1 and it was able to release VPA slowly within the brain. Therefore, it was concluded that the VPA brain concentrations can be stabilized as well as the problematic pharmacokinetic profile can be altered by a LAT1-selective prodrug.

Brain pharmacokinetics of ganciclovir in rats with orthotopic BT4C glioma.

Ganciclovir (GCV) is an essential part of the Herpes simplex virus thymidine kinase (HSV-tk) gene therapy of malignant gliomas. The purpose of this study was to investigate the brain pharmacokinetics and tumor uptake of GCV in the BT4C rat glioma model. GCV's brain and tumor uptakes were investigated by in vivo microdialysis in rats with orthotopic BT4C glioma. In addition, the ability of GCV to cross the blood-brain barrier and tumor vasculature was assessed with in situ rat brain perfusion. Finally, the extent to which GCV could permeate across the BT4C glioma cell membrane was assessed in vitro. The areas under the concentration curve of unbound GCV in blood, brain extracellular fluid (ECF), and tumor ECF were 6157, 1658, and 4834 µM⋅min, respectively. The apparent maximum unbound concentrations achieved within 60 minutes were 46.9, 11.8, and 25.8 µM in blood, brain, and tumor, respectively. The unbound GCV concentrations in brain and tumor after in situ rat brain perfusion were 0.41 and 1.39 nmol/g, respectively. The highly polar GCV likely crosses the fenestrated tumor vasculature by paracellular diffusion. Thus, GCV is able to reach the extracellular space around the tumor at higher concentrations than that in healthy brain. However, GCV uptake into BT4C cells at 100 µM was only 2.1 pmol/mg of protein, and no active transporter-mediated disposition of GCV could be detected in vitro. In conclusion, the limited efficacy of HSV-tk/GCV gene therapy may be due to the poor cellular uptake and rapid elimination of GCV.

Quantitative insight into the design of compounds recognized by the L-type amino acid transporter 1 (LAT1).

L-Type amino acid transporter 1 (LAT1) is a transmembrane protein expressed abundantly at the blood-brain barrier (BBB), where it ensures the transport of hydrophobic acids from the blood to the brain. Due to its unique substrate specificity and high expression at the BBB, LAT1 is an intriguing target for carrier-mediated transport of drugs into the brain. In this study, a comparative molecular field analysis (CoMFA) model with considerable statistical quality (Q(2) =0.53, R(2) =0.75, Q(2) SE=0.77, R(2) SE=0.57) and good external predictivity (CCC=0.91) was generated. The model was used to guide the synthesis of eight new prodrugs whose affinity for LAT1 was tested by using an in situ rat brain perfusion technique. This resulted in the creation of a novel LAT1 prodrug with L-tryptophan as the promoiety; it also provided a better understanding of the molecular features of LAT1-targeted high-affinity prodrugs, as well as their promoiety and parent drug. The results obtained will be beneficial in the rational design of novel LAT1-binding prodrugs and other compounds that bind to LAT1.

Sustained release of metformin via red blood cell accumulated sulfenamide prodrug.

Metformin is a first-line antidiabetic drug to treat type 2 diabetes. It is rapidly eliminated from plasma but also accumulated into red blood cells (RBCs) from which it is slowly released back into plasma. The aim of the study was to evaluate whether the amount of metformin in the RBCs could be increased by a sulfenamide prodrug approach, which could provide longer duration of metformin in systemic circulation. Pharmacokinetic properties of metformin and its cyclohexyl sulfenamide prodrug were evaluated in plasma and in whole blood after intravenous and oral administration in rats. Once the sulfenamide prodrug reached the bloodstream, it was rapidly and efficiently accumulated into the RBCs, where it was converted to metformin by free thiols. The RBC-whole blood ratio of metformin was increased approximately from 42% to 96% when metformin was administered intravenously as its sulfenamide prodrug, and the proportion of metformin in the RBCs was found to be concentration and time independent. Because metformin was slowly liberated into plasma, the prodrug showed a sustained-release pharmacokinetic profile and longer plasma half-life for metformin after oral administration. Therefore, this sulfenamide prodrug has great potential to improve metformin therapy as the daily doses could be reduced.

Design, synthesis and brain uptake of LAT1-targeted amino acid prodrugs of dopamine.

PURPOSE: Drug delivery to the brain is impeded by the blood-brain barrier (BBB). Here, we attempted to enhance the brain uptake of cationic dopamine by utilizing the large amino acid transporter 1 (LAT1) at the BBB by prodrug approach. METHODS: Three amino acid prodrugs of dopamine were synthesized and their prodrug properties were examined in vitro. Their LAT1-binding and BBB-permeation were studied using the in situ rat brain perfusion technique. The brain uptake after intravenous administration and the dopamine-releasing ability in the rat striatum after intraperitoneal administration were also determined for the most promising prodrug. RESULTS: All prodrugs underwent adequate cleavage in rat tissue homogenates. The prodrug with phenylalanine derivative as the promoiety had both higher affinity for LAT1 and better brain uptake properties than those with an alkyl amino acid - mimicking promoiety. The phenylalanine prodrug was taken up into the brain after intravenous injection but after intraperitoneal injection the prodrug did not elevate striatal dopamine concentrations above those achieved by corresponding L-dopa treatment. CONCLUSIONS: These results indicate that attachment of phenylalanine to a cationic drug via an amide bond from the meta-position of its aromatic ring could be highly applicable in prodrug design for LAT1-mediated CNS-delivery of not only anionic but also cationic polar drugs.

Structure-activity relationship study of compounds binding to large amino acid transporter 1 (LAT1) based on pharmacophore modeling and in situ rat brain perfusion.

Large neutral amino acid transporter 1 (LAT1) is predominantly expressed at the blood-brain barrier and it has a major role in transporting neutral amino acids into the brain. LAT1 has the potential to function as a drug carrier for improved drug brain delivery which makes it an intriguing target protein for central nervous system disorders, e.g., Alzheimer's disease, Parkinson's disease and brain tumors. In this study, a 3D pharmacophore was generated for a set of LAT1 substrates whose binding affinities were studied using competitive inhibition of the brain uptake of [¹4C]-L-leucine with an in situ rat brain perfusion method. The pharmacophore highlights the most important molecular features shared by efficient LAT1-binding compounds and elucidates their 3D-arrangement in detail. This clarifies the structure-activity relationships of LAT1 substrates and provides insights for making a binding hypothesis. The results can be further applied in the design of novel efficient LAT1 substrates.

Large amino acid transporter 1 (LAT1) prodrugs of valproic acid: new prodrug design ideas for central nervous system delivery.

Central nervous system (CNS) drug delivery is a major challenge in drug development because the blood-brain barrier (BBB) efficiently restricts the entry of drug molecules into the CNS at sufficient amounts. The brain uptake of poorly penetrating drugs could be improved by utilizing the transporters at the BBB with a prodrug approach. In this study, we designed four phenylalanine derivatives of valproic acid and studied their ability to utilize a large amino acid transporter 1 (LAT1) in CNS delivery with an aim to show that the meta-substituted phenylalanine prodrugs bind to LAT1 with a higher affinity compared with the affinity of the para-substituted derivatives. All of the prodrugs crossed the BBB carrier mediatedly via LAT1 in in situ rat brain perfusion. For the first time, we introduced a novel meta-substituted phenylalanine analogue promoiety which improved the LAT1 affinity 10-fold and more importantly the rat brain uptake of the prodrug 2-fold compared with those of the para-substituted derivatives. Therefore, we have characterized a new prodrug design idea for CNS drug delivery utilizing a transporter-mediated prodrug approach.

[Prodrug technology in the development of medicinal agents]. / Aihiolääketeknologia lääkekehityksessä.

Medicinal agents acetosalicylic acid, lansoprazole, simvastatin, clopidogrel and oseltamivir are known prodrugs. Prodrugs are pharmacologically inactive medicine molecules, from which the active drug is released in the body through a chemical or enzymatic reaction. This mechanism can be used to solve pharmaceutical and pharmacokinetic problems restricting the use of medicinal agents, without altering the pharmacological effects of the agent that ends up in the site of action. Of the medicinal agents currently on the market, as many as one out of ten is a prodrug.

Glucose promoiety enables glucose transporter mediated brain uptake of ketoprofen and indomethacin prodrugs in rats.

The brain uptake of solutes is efficiently governed by the blood-brain barrier (BBB). The BBB expresses a number of carrier-mediated transport mechanisms, and new knowledge of these BBB transporters can be used in the rational targeted delivery of drug molecules for active transport. One attractive approach is to conjugate an endogenous transporter substrate to the active drug molecule to utilize the prodrug approach. In the present study, ketoprofen and indomethacin were conjugated with glucose and the brain uptake mechanism of the prodrugs was determined with the in situ rat brain perfusion technique. Two of the prodrugs were able to significantly inhibit the uptake of glucose transporter (GluT1)-mediated uptake of glucose, thereby demonstrating affinity to the transporter. Furthermore, the prodrugs were able to cross the BBB in a temperature-dependent manner, suggesting that the brain uptake of the prodrugs is carrier-mediated.