Stereoselective Evasion of P-glycoprotein, Cytochrome P450 3A, and Hydrolases by Peptide Prodrug Modification of Saquinavir

Exploration of in vitro drug release testing methods for saquinavir microenvironmental pH modifying buccal films

Buccal films containing a pH modifying excipient may be able to increase bioavailability of drugs with pH-dependent solubility such as saquinavir. Access to suitable in vitro drug release testing methods may facilitate buccal formulation development. This study aimed to explore two release testing methods for characterising buccal films and to elucidate the relationship between microenvironmental pH (pH_M, i.e. the pH around the swelling films) and saquinavir release. The Franz diffusion cell method was applicable to investigate the effect of hydroxypropyl methylcellulose (HPMC) grade on saquinavir release. Films containing HPMC K3 LV had a faster saquinavir release than films containing HPMC K100 LV. A UV/Vis imaging method was developed to visualise saquinavir release and pH_M changes during the initial dissolution. Within 5 min, the pH_M decreased from 6.8 to around 5.4 for HPMC K100 LV-based films containing 11.1 % or 16.6 % (w/w) malic acid. Subsequently, the pH_M increased due to increasing concentrations of saquinavir. An increase in malic acid content led to a faster saquinavir release. The combination of methods may be broadly applicable for excipient screening in development of buccal formulations. The imaging approach holds promise for characterizing other pH modifying formulation principles.

Considerations for Determining Direct Versus Indirect Functional Effects of Solubilizing Excipients on Drug Transporters for Enhancing Bioavailability

Excipients used in drug formulations at clinically safe levels have been considered to be pharmacologically inert; however, numerous studies have suggested that many solubilizing agents may modulate drug transporter activities and intestinal absorption. Here, the reported interactions between various solubilizing excipients and drug transporters are evaluated to consider various potential underlying mechanisms. This forms the basis for debate in the field in regard to whether or not the effects are based on "direct" interactions or "indirect" consequences arising from the role of the excipients. For example, an increase in apparent drug solubility can give rise to saturation of transporters according to Michaelis-Menten kinetics. This is also drawing the attention of regulatory agencies as they seek to understand the role of formulation additives. The continued application of excipients as a tool in solubility enhancement is crucial in the drug development process, creating a need for additional data to verify the proposed mechanism behind these changes. A literature review is provided here with some guidance on other factors that should be considered to delineate the effects that arise from direct physiological interactions or indirect effects. The results of such studies may aid the rational design of bioavailability-enhancing formulations.

Enhancement of saquinavir absorption and accumulation through the formation of solid drug nanoparticles

Background

Nanotechnology is now considered a promising drug delivery method for orally administered hydrophobic drugs to their sites of action. The effect of nanodispersion on cellular transport and accumulation of saquinavir (SQV) was investigated.

Methods

The transport of five solid drug nanoparticle (SDN) SQV formulations along Caco-2 cell monolayers (CCM) was compared to that of standard SQV. The SDNs were prepared using SQV mesylate (20%), Pluronic F127 (10%) plus five other excipients (HPMC, PVP, PVA, Lecithin S75 and Span 80) in different proportions. Cellular accumulation in CEM parental and CEMVBL (P-gp overexpressing) cells was conducted to ascertain the effect of nanodispersion on P-gp mediated efflux of SQV. All SDN formulations were dissolved in water, whereas SQV in DMSO to improve solubility. Quantification was via HPLC.

Results

From transport results, an SDN sample composed of SQV mesylate/Pluronic F127 plus HPMC (70%) and had a 24% increase in apparent absorption compared to standard SQV, largely driven by a 38% reduction in basolateral to apical permeation. Additionally, the formulation and two others (SQV mesylate/Pluronic F127 alone; and + HPMC (65%)/Lecithin [5%]) accumulated more significantly in CEM cells, suggesting enhanced delivery to these cells. Moreover, accumulation and transport of the three SDNs compared well to that of SQV despite being dissolved in water, suggestive of improved dissolution. The inclusion of PVA resulted in increased efflux.

Conclusion

The use of HPMC and Pluronic F127 produced SQV SDNs with improved permeation in Caco-2 cells and improved accumulation in CEM cells, but negative effects with PVA.

Coupling of an Acyl Migration Prodrug Strategy with Bio-activation To Improve Oral Delivery of the HIV-1 Protease Inhibitor Atazanavir

HIV-1 protease inhibitors (PIs), which include atazanavir (ATV, 1), remain important medicines to treat HIV-1 infection. However, they are characterized by poor oral bioavailability and a need for boosting with a pharmacokinetic enhancer, which results in additional drug-drug interactions that are sometimes difficult to manage. We investigated a chemo-activated, acyl migration-based prodrug design approach to improve the pharmacokinetic profile of 1 but failed to obtain improved oral bioavailability over dosing the parent drug in rats. This strategy was refined by conjugating the amine with a promoiety designed to undergo bio-activation, as a means of modulating the subsequent chemo-activation. This culminated in a lead prodrug that (1) yielded substantially better oral drug delivery of 1 when compared to the parent itself, the simple acyl migration-based prodrug, and the corresponding simple l-Val prodrug, (2) acted as a depot which resulted in a sustained release of the parent drug in vivo, and (3) offered the benefit of mitigating the pH-dependent absorption associated with 1, thereby potentially reducing the risk of decreased bioavailability with concurrent use of stomach-acid-reducing drugs.

Design strategies in the prodrugs of HIV-1 protease inhibitors to improve the pharmaceutical properties

Combination antiretroviral therapy (cART) is currently the most effective treatment for HIV-1 infection. HIV-1 protease inhibitors (PIs) are an important component of some regimens of cART. However, PIs are known for sub-optimal ADME properties, resulting in poor oral bioavailability. This often necessitates high drug doses, combination with pharmacokinetic enhancers and/or special formulations in order to effectively deliver PIs, which may lead to a high pill burden and reduced patient compliance. As a remedy, improving the ADME properties of existing drugs via prodrug and other approaches has been pursued in addition to the development of next generation PIs with improved pharmacokinetic, resistance and side effect profiles. Phosphate prodrugs have been explored to address the solubility-limiting absorption and high excipient load. Prodrug design to target carrier-mediated drug delivery has also been explored. Amino acid prodrugs have been shown to improve permeability by engaging active transport mechanisms, reduce efflux and mitigate first pass metabolism while acyl migration prodrugs have been shown to improve solubility. Prodrug design efforts have led to the identification of one marketed agent, fosamprenavir, and clinical studies with two other prodrugs. Several of the reported approaches lack detailed in vivo characterization and hence the potential preclinical or clinical benefits of these approaches are yet to be fully determined.

Circumvention of P-gp and MRP2 mediated efflux of lopinavir by a histidine based dipeptide prodrug

PURPOSE

This study was aimed to develop a novel Histidine-Leucine-Lopinavir (His-Leu-LPV) dipeptide prodrug and evaluate its potential for circumvention of P-gp and MRP2-mediated efflux of lopinavir (LPV) indicated for HIV-1 infection.

METHODS

His-Leu-LPV was synthesized following esterification of hydroxyl group of LPV and was identified by (1)H NMR and LCMS/MS techniques. Aqueous solubility, stability and cell cytotoxicity of prodrug was determined. Uptake and permeability studies were carried out using P-gp (MDCK-MDR1) and MRP2 (MDCK-MRP2) transfected cell lines. To further delineate prodrug uptake, prodrug interaction with influx transporters (PepT1 and PHT1) was determined. Enzymatic hydrolysis and reconversion of His-Leu-LPV to LPV was examined using Caco-2 cell homogenates.

RESULTS

Aqueous solubility generated by the prodrug was markedly higher relative to unmodified LPV. Importantly, His-Leu-LPV displayed significantly lower affinity towards P-gp and MRP2 as evident from higher uptake and transport rates. [3H]-GlySar and [3H]-l-His uptake receded to approximately 30% in the presence of His-Leu-LPV supporting the PepT1/PHT1 mediated uptake process. A steady regeneration of LPV and Leu-LPV in Caco-2 cell homogenates indicated His-Leu-LPV undergoes both esterase and peptidase-mediated hydrolysis.

CONCLUSION

Histidine based dipeptide prodrug approach can be an alternative strategy to improve LPV absorption across poorly permeable intestinal barrier.

Stereoisomeric Prodrugs to Improve Corneal Absorption of Prednisolone: Synthesis and In Vitro Evaluation

A series of stereoisomeric prodrugs have been designed to examine efficacy in generating higher corneal absorption relative to prednisolone. Prodrugs have been studied and identified with LC/MS/MS and NMR analyses. Prodrugs have been characterized for aqueous solubility, buffer stability, and cytotoxicity. Cellular uptake and permeability studies have been conducted across MDCK-MDR1 cells to determine prodrug affinity towards P-glycoprotein (P-gp) and peptide transporters. Enzyme-mediated degradation of prodrugs has been determined using Statens Seruminstitut rabbit cornea (SIRC) cell homogenates. Prodrugs exhibited higher aqueous solubility relative to prednisolone. Prodrugs circumvented P-gp-mediated cellular efflux and were recognized by peptide transporters. Prodrugs (DP, DDP) produced with D-isomers (D-valine) were significantly stable against both chemical and enzymatic hydrolyses. The order of degradation rate constants observed in chemical and enzymatic hydrolyses were in the same order, i.e., L-valine-L-valine-prednisolone (LLP) > L-valine-D-valine-prednisolone (LDP) > D-valine-L-valine-prednisolone (DLP) > D-valine-D-valine-prednisolone (DDP). Results obtained from this study clearly suggest that stereoisomeric prodrug approach is an effective strategy to overcome P-gp-mediated efflux and improve transcorneal permeability of prednisolone following topical administration.

Prodrug approach to improve absorption of prednisolone

Amino acid and dipeptide prodrugs have been developed to examine their potential in enhancing aqueous solubility and permeability as well as to bypass P-glycoprotein (P-gp) mediated cellular efflux of prednisolone. Prodrugs have been synthesized and identified with LC/MS/MS and NMR. Prodrugs displayed significantly higher aqueous solubility relative to prednisolone. These compounds also exhibited higher stability under acidic conditions relative to basic medium. [14]-Erythromycin uptake remained unaltered in the presence of valine-valine-prednisolone (VVP) indicating lower affinity toward P-gp. Moreover, VVP generated significantly higher transepithelial permeability across MDCK-MDR1 cells compared to prednisolone. Importantly, [3H]-GlySar uptake diminished significantly in the presence of VVP indicating high affinity toward peptide transporters. Moreover, prednisolone was regenerated from VVP due to enzymatic hydrolysis in SIRC cell homogenate. Results obtained from these studies clearly suggest that peptide transporter targeted prodrugs is a viable strategy to improve aqueous solubility and overcome P-gp mediated cellular efflux of prednisolone.

Dipeptide prodrug approach to evade efflux pumps and CYP3A4 metabolism of lopinavir

Oral absorption of lopinavir (LPV) is limited due to P-glycoprotein (P-gp) and multidrug resistance-associated protein2 (MRP2) mediated efflux by intestinal epithelial cells. Moreover, LPV is extensively metabolized by CYP3A4 enzymes. In the present study, dipeptide prodrug approach was employed to circumvent efflux pumps (P-gp and MRP2) and CYP3A4 mediated metabolism of LPV. Valine-isoleucine-LPV (Val-Ile-LPV) was synthesized and identified by LCMS and NMR techniques. The extent of LPV and Val-Ile-LPV interactions with P-gp and MRP2 was studied by uptake and transport studies across MDCK-MDR1 and MDCK-MRP2 cells. To determine the metabolic stability, time and concentration dependent degradation study was performed in liver microsomes. Val-Ile-LPV exhibited significantly higher aqueous solubility relative to LPV. This prodrug generated higher stability under acidic pH. Val-Ile-LPV demonstrated significantly lower affinity toward P-gp and MRP2 relative to LPV. Transepithelial transport of Val-Ile-LPV was significantly higher in the absorptive direction (apical to basolateral) relative to LPV. Importantly, Val-Ile-LPV was recognized as an excellent substrate by peptide transporter. Moreover, Val-Ile-LPV displayed significantly higher metabolic stability relative to LPV. Results obtained from this study suggested that dipeptide prodrug approach is a viable option to elevate systemic levels of LPV following oral administration.

Amino Acid Prodrugs: An Approach to Improve the Absorption of HIV-1 Protease Inhibitor, Lopinavir

Poor systemic concentrations of lopinavir (LPV) following oral administration occur due to high cellular efflux by P-glycoprotein (P-gp) and multidrug resistance-associated proteins (MRPs) and extensive metabolism by CYP3A4 enzymes. In this study, amino acid prodrugs of LPV were designed and investigated for their potential to circumvent efflux processes and first pass effects. Three amino acid prodrugs were synthesized by conjugating isoleucine, tryptophan and methionine to LPV. Prodrug formation was confirmed by the LCMS/MS and NMR technique. Interaction of LPV prodrugs with efflux proteins were carried out in P-gp (MDCK-MDR1) and MRP2 (MDCK-MRP2) transfected cells. Aqueous solubility studies demonstrated that prodrugs generate higher solubility relative to LPV. Prodrugs displayed higher stability under acidic conditions and degraded significantly with rise in pH. Uptake and transport data suggested that prodrugs carry significantly lower affinity towards P-gp and MRP2 relative to LPV. Moreover, prodrugs exhibited higher liver microsomal stability relative to LPV. Hence, amino acid prodrug modification might be a viable approach for enhancing LPV absorption across intestinal epithelial and brain endothelial cells which expresses high levels of P-gp and MRP2.

The dipeptide monoester prodrugs of floxuridine and gemcitabine-feasibility of orally administrable nucleoside analogs

Dipeptide monoester prodrugs of floxuridine and gemcitabine were synthesized. Their chemical stability in buffers, enzymatic stability in cell homogenates, permeability in mouse intestinal membrane along with drug concentration in mouse plasma, and anti-proliferative activity in cancer cells were determined and compared to their parent drugs. Floxuridine prodrug was more enzymatically stable than floxuridine and the degradation from prodrug to parent drug works as the rate-limiting step. On the other hand, gemcitabine prodrug was less enzymatically stable than gemcitabine. Those dipeptide monoester prodrugs exhibited 2.4- to 48.7-fold higher uptake than their parent drugs in Caco-2, Panc-1, and AsPC-1 cells. Floxuridine and gemcitabine prodrugs showed superior permeability in mouse jejunum to their parent drugs and exhibited the higher drug concentration in plasma after in situ mouse perfusion. Cell proliferation assays in ductal pancreatic cancer cells, AsPC-1 and Panc-1, indicated that dipeptide prodrugs of floxuridine and gemcitabine were more potent than their parent drugs. The enhanced potency of nucleoside analogs was attributed to their improved membrane permeability. The prodrug forms of 5′-l-phenylalanyl-l-tyrosyl-floxuridine and 5′-l-phenylalanyl-l-tyrosyl-gemcitabine appeared in mouse plasma after the permeation of intestinal membrane and the first-pass effect, suggesting their potential for the development of oral dosage form for anti-cancer agents.

Influence of overexpression of efflux proteins on the function and gene expression of endogenous peptide transporters in MDR-transfected MDCKII cell lines

The objective of this study is to delineate whether overexpression of human efflux transporters (P-gp, MRP2, and BCRP) in transfected MDCK cells affect the functional activities, and gene and protein expression of endogenous influx peptide transporter system (PepT). Real-time PCR, immunoblotting, uptake and permeability studies of [(3)H]Gly-Sar were conducted on transfected MDCKII and wild-type cells to investigate functional differences. Cellular [(3)H]Gly-Sar accumulation was significantly lower in transfected MDCKII cell lines compared to wild-type cells. Transport efficiency of apical peptide transporters was markedly reduced to around 25%, 30%, and 40% in P-gp-, MRP2-, and BCRP-overexpressed MDCK cell lines, respectively. With ascending cell-passage, transport efficiency was enhanced. A significantly higher Gly-Sar permeability was observed across parental cell-monolayers over transfected cells at all pHs. Levels of mRNA for both canine PepT1 and PepT2 were substantially reduced when efflux transporters overexpressed but enhanced when mRNA-levels of efflux genes diminished with ascending cell-passage of transfected cells. An inverse correlation was evident between endogenous PepT and exogenous efflux transporters in transfected MDCKII cells. Results of protein expression also supported these findings. Overexpression of MDR genes can affect endogenous PepT function which might be due to the phenomenon of transporter-compensation resulting in down-regulation of endogenous genes.