Structure activity relationship of selective GABA uptake inhibitors.

A series of ß-amino acids with lipophilic diaromatic side chain was synthesized and characterized pharmacologically on mouse γ-amino butyric acid (GABA) transporter subtypes mGAT1-4 in order to investigate structure activity relationships (SAR) for mGAT2 (corresponding to hBGT-1). Variation in the lipophilic diaromatic side chain was probed to understand the role of the side chain for activity. This yielded several selective compounds of which the best (1R,2S)-5a was more than 10 fold selective towards other subtypes, although potency was moderate. A docking study was performed to investigate possible binding modes of the compounds in mGAT2 suggesting a binding mode similar to that proposed for Tiagabine in hGAT1. Specific interactions between the transporter and the amino acid part of the ligands may account for a reverted preference towards mGAT2 over mGAT1.

Using in vitro lipolysis and SPECT/CT in vivo imaging to understand oral absorption of fenofibrate from lipid-based drug delivery systems.

Using lipid-based drug delivery systems (LbDDS) is an efficient strategy to enhance the low oral bioavailability of poorly water-soluble drugs. Here the oral absorption of fenofibrate (FF) from LbDDS in rats was investigated in pharmacokinetic, in vitro lipolysis, and SPECT/CT in vivo imaging studies. The investigated formulations were soybean oil solution (SBO), a mixture of soybean oil and monoacyl phosphatidylcholine (MAPC) (SBO-MAPC), self-nanoemulsifying drug delivery systems with and without MAPC (SNEDDS-MAPC and SNEDDS, respectively), and an aqueous suspension (SUSP) as a reference. Oral bioavailability of the LbDDS ranged from 27 to 35%. A two-step in vitro lipolysis model simulating rat gastro-intestinal digestion provided in vitro FF solubilisation data to understand oral absorption. During the in vitro lipolysis, most FF was undissolved for SUSP and distributed into the poorly dispersed oil phase for SBO. For the SNEDDS without MAPC, practically all FF solubilised into the aqueous phase during the dispersion and digestion. Adding MAPC to SBO enhanced the dispersion of the oil phase into the digestion media while adding MAPC to SNEDDS resulted in a distribution of 29% of FF into the oil phase at the beginning of in vitro lipolysis. FF distribution into both oil and aqueous phases explained the higher and prolonged oral absorption of LbDDS containing MAPC. To elucidate the relatively low bioavailability of all formulations, FF and triolein were labeled with 123I and 125I, respectively, to study the biodistribution of drug and lipid excipients in a dual isotope SPECT/CT in vivo imaging study. The concentration of radiolabeled drug as a function of time in the heart correlated to the plasma curves. A significant amount of radiolabeled drug and lipids (i.e., 28-59% and 24-60% of radiolabeled drug and lipids, respectively) was observed in the stomach at 24 h post administration, which can be linked to the low bioavailability of the formulations. The current study for the first time combined in vitro lipolysis and dual isotope in vivo imaging to find the root cause of different fenofibrate absorption profiles from LbDDS and an aqueous suspension.

Challenges and trends in apomorphine drug delivery systems for the treatment of Parkinson's disease.

Parkinson's disease (PD) is a chronic debilitating disease affecting approximately 1% of the population over the age of 60. The severity of PD is correlated to the degree of dopaminergic neuronal loss. Apomorphine has a similar chemical structure as the neurotransmitter dopamine and has been used for the treatment of advanced PD patients. In PD patients, apomorphine is normally administered subcutaneously with frequent injections because of the compound's extensive hepatic first-pass metabolism. There is, hence, a large unmet need for alternative administrative routes for apomorphine to improve patient compliance. The present review focuses on the research and development of alternative delivery of apomorphine, aiming to highlight the potential of non-invasive apomorphine therapy in PD, such as sublingual delivery and transdermal delivery.

Efficacy of oral lipid-based formulations of apomorphine and its diester in a Parkinson's disease rat model.

OBJECTIVES: Apomorphine is used to symptomatically treat Parkinson's disease (PD). Oral delivery of apomorphine is generally limited by its short plasma half-life and a hepatic first-pass metabolism. This study was aimed at evaluating the behavioural response of apomorphine and its prodrug administered in oral lipid-based formulations. METHODS: The behavioural response of apomorphine and its prodrug administered in oral lipid-based formulations was evaluated using a 6-hydroxydopamine-lesioned rat model simulating PD symptomatology. Apomorphine or dipalmitoyl apomorphine (DPA) was incorporated into different lipid-based formulations and orally administered (0.24 mmol/kg) to the PD rat model. The rotations by the rats were counted. KEY FINDINGS: The duration of response lasted to about 2.5 h with oral apomorphine- and DPA-loaded o/w emulsion, while it was increased to 6 h when DPA was incorporated in self-emulsifying drug delivery systems compared to s.c. apomorphine (1 h). This suggests that the lipid-based formulations provide a sustained drug release allowing for a steady exposure to the brain. CONCLUSIONS: Oral lipid-based apomorphine delivery has a potential in achieving a steady response, though at a higher dose possibly eliminating the need for frequent s.c. apomorphine administration.

Structure-Activity Relationship, Pharmacological Characterization, and Molecular Modeling of Noncompetitive Inhibitors of the Betaine/γ-Aminobutyric Acid Transporter 1 (BGT1).

N-(1-Benzyl-4-piperidinyl)-2,4-dichlorobenzamide 5 (BPDBA) is a noncompetitive inhibitor of the betaine/GABA transporter 1 (BGT1). We here report the synthesis and structure-activity relationship of 71 analogues. We identify 26m as a more soluble 2,4-Cl substituted 3-pyridine analogue with retained BGT1 activity and an improved off-target profile compared to 5. We performed radioligand-based uptake studies at chimeric constructs between BGT1 and GAT3, experiments with site-directed mutated transporters, and computational docking in a BGT1 homology model based on the newly determined X-ray crystal structure of the human serotonin transporter (hSERT). On the basis of these experiments, we propose a binding mode involving residues within TM10 in an allosteric site in BGT1 that corresponds to the allosteric binding pocket revealed by the hSERT crystal structure. Our study provides first insights into a proposed allosteric binding pocket in BGT1, which accommodates the binding site for a series of novel noncompetitive inhibitors.

In vivo evaluation of lipid-based formulations for oral delivery of apomorphine and its diester prodrugs.

In the present study, the differences in oral absorption of apomorphine and its diester prodrugs and the effect of lipid-based formulations on the absorption of apomorphine or its prodrugs were investigated. Apomorphine, dilauroyl apomorphine (DLA) and dipalmitoyl apomorphine (DPA) were orally administered (0.24mmol/kg) to rats as: DLA-o/w emulsion, DPA-o/w emulsion, apomorphine-o/w emulsion, apomorphine aqueous suspension, DLA-Maisine, DLA-soybean oil, DLA-self-emulsifying drug delivery systems (SEDDS), and DLA-w/o emulsion. The o/w and w/o emulsion consisted of Maisine 35-1 emulsified with water in 1:3 and 4:1 ratio, respectively. Tmax of diesters was significantly increased (p≤0.05) compared to apomorphine in o/w emulsion, suggesting that esterification yielded prolonged drug absorption. Cmax, AUC and the relative bioavailability of apomorphine after DLA-SEDDS administration was higher (p≤0.05) than after DLA-w/o administration, indicating that triglycerides and surfactants improved the oral absorption of DLA. Similarly, Cmax and AUC after dosing apomorphine-o/w were significantly higher (p≤0.05) than that of aqueous suspension. This suggested that lipids and lipolysis products possibly aided apomorphine micellar solubilization in intestinal fluids. A combination of prodrug strategy and lipid-based formulations facilitated a higher and prolonged absorption of apomorphine from its diester prodrugs.

Apomorphine and its esters: Differences in Caco-2 cell permeability and chylomicron affinity.

Oral delivery of apomorphine via prodrug principle may be a potential treatment for Parkinson's disease. The purpose of this study was to investigate the transport and stability of apomorphine and its esters across Caco-2 cell monolayer and their affinity towards chylomicrons. Apomorphine, monolauroyl apomorphine (MLA) and dilauroyl apomorphine (DLA) were subjected to apical to basolateral (A-B) and basolateral to apical (B-A) transport across Caco-2 cell monolayer. The stability of these compounds was also assessed by incubation at intestinal pH and physiological pH with and without Caco-2 cells. Molecular dynamics (MD) simulations were performed to understand the stability of the esters on a molecular level. The affinity of the compounds towards plasma derived chylomicrons was assessed. The A-B transport of intact DLA was about 150 times lower than the transport of apomorphine. In contrast, MLA was highly unstable in the aqueous media leading to apomorphine appearance basolaterally. MD simulations possibly explained the differences in hydrolysis susceptibilities of DLA and MLA. The affinity of apomorphine diesters towards plasma derived chylomicrons provided an understanding of their potential lymphatic transport. The intact DLA transport is not favorable; therefore, the conversion of DLA to MLA is an important step for intestinal apomorphine absorption.

Lipophilic prodrugs of apomorphine I: preparation, characterisation, and in vitro enzymatic hydrolysis in biorelevant media.

Apomorphine, a subcutaneously administered drug for Parkinson's disease with short half-life requires frequent administration leading to patient non-compliance. This study aimed at synthesising and purifying lipophilic diesters of apomorphine, and investigating their in vitro degradation in biorelevant media before and after incorporating them into self-emulsifying drug delivery systems (SEDDS) for oral delivery. Two apomorphine diester prodrugs were synthesised: dilauroyl apomorphine (DLA) and dipalmitoyl apomorphine (DPA). The in vitro enzymatic hydrolysis of diesters was performed using biorelevant media with pancreatin to catalyse the diester degradation. The synthesised and purified diesters were found to be free from reactants as impurities confirmed by LC/MS and NMR. DLA and DPA were degraded into corresponding monoesters and free apomorphine within 5 min after adding pancreatin, leaving about 4% and 28% of the intact diester, respectively. The incorporation of the diesters into SEDDS reduced the enzymatic degradation of diesters. In addition, the chain length of diester and the type of oil used in formulations affected diester hydrolysis. The lipophilic apomorphine diesters were substrates of lipases present in pancreatin, and the degree of diester degradation can be controlled by selecting suitable lipid excipients. Therefore, diesters of apomorphine are promising prodrugs for oral delivery aiming at lymphatic transport.

Investigating the correlation between in vivo absorption and in vitro release of fenofibrate from lipid matrix particles in biorelevant medium.

Lipid matrix particles (LMP) may be used as better carriers for poorly water-soluble drugs than liquid lipid carriers because of reduced drug mobilization in the formulations. However, the digestion process of solid lipid particles and their effect on the absorption of poorly water-soluble drugs are not fully understood. This study aimed at investigating the effect of particle size of LMP on drug release in vitro as well as absorption in vivo in order to get a better understanding on the effect of degradation of lipid particles on drug solubilisation and absorption. Fenofibrate, a model poorly water-soluble drug, was incorporated into LMP in this study using probe ultrasound sonication. The resultant LMP were characterised in terms of particle size, size distribution, zeta potential, entrapment efficiency, in vitro lipolysis and in vivo absorption in rat model. LMP of three different particle sizes i.e. approximately 100 nm, 400 nm, and 10 µm (microparticles) were produced with high entrapment efficiencies. The in vitro lipolysis study showed that the recovery of fenofibrate in the aqueous phase for 100 nm and 400 nm LMP was significantly higher (p<0.05) than that of microparticles after 30 min of lipolysis, suggesting that nano-sized LMP were digested to a larger extent due to greater specific surface area. The 100 nm LMP showed faster initial digestion followed by 400 nm LMP and microparticles. The area under the plasma concentration-time curve (AUC) following oral administration of 100 nm LMP was significantly higher (p<0.01) than that of microparticles and fenofibrate crystalline suspension (control). However, no significant difference was observed between the AUCs of 100 nm and 400 nm LMP. The same rank order on the in vivo absorption and the in vitro response was observed. The recovery (%) of fenofibrate partitioning into the aqueous phase during in vitro lipolysis and the AUC of plasma concentration-time curve of fenofibric acid was in the order of 100 nm LMP>microparticles>control. In summary, the present study demonstrated the particle size dependence of bioavailability of fenofibrate loaded LMP in rat model which correlates well with the in vitro drug release performed in the biorelevant medium.

Selective mGAT2 (BGT-1) GABA uptake inhibitors: design, synthesis, and pharmacological characterization.

ß-Amino acids sharing a lipophilic diaromatic side chain were synthesized and characterized pharmacologically on mouse GABA transporter subtypes mGAT1-4. The parent amino acids were also characterized. Compounds 13a, 13b, and 17b displayed more than 6-fold selectivity for mGAT2 over mGAT1. Compound 17b displayed anticonvulsive properties inferring a role of mGAT2 in epileptic disorders. These results provide new neuropharmacological tools and a strategy for designing subtype selective GABA transport inhibitors.