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

A deep eutectic-based, self-emulsifying subcutaneous depot system for apomorphine therapy in Parkinson's disease

Parkinson’s disease (PD) is a progressive disorder of the central nervous system that affects motor control. While subcutaneous injection of apomorphine (Apokyn) is clinically used to alleviate intermittent episodes of dyskinesia, the treatment requires multiple injections of the drug per day, significantly deterring patient compliance. We introduce a deep eutectic-based ternary solvent system that self-emulsifies in situ following subcutaneous injection and entraps apomorphine, allowing a 48-h duration of detectable drug concentration in the plasma of pigs, which is a remarkable improvement over the clinical comparator. The results from the animal studies support the self-emulsifying system as a potent, long-lasting therapeutic for PD patients and potentially for other therapeutics that have a similar delivery challenge.

Apomorphine, a dopamine agonist, is a highly effective therapeutic to prevent intermittent off episodes in advanced Parkinson’s disease. However, its short systemic half-life necessitates three injections per day. Such a frequent dosing regimen imposes a significant compliance challenge, especially given the nature of the disease. Here, we report a deep eutectic-based formulation that slows the release of apomorphine after subcutaneous injection and extends its pharmacokinetics to convert the current three-injections-a-day therapy into an every-other-day therapy. The formulation comprises a homogeneous mixture of a deep eutectic solvent choline-geranate, a cosolvent n-methyl-pyrrolidone, a stabilizer polyethylene glycol, and water, which spontaneously emulsifies into a microemulsion upon injection in the subcutaneous space, thereby entrapping apomorphine and significantly slowing its release. Ex vivo studies with gels and rat skin demonstrate this self-emulsification process as the mechanism of action for sustained release. In vivo pharmacokinetics studies in rats and pigs further confirmed the extended release and improvement over the clinical comparator Apokyn. In vivo pharmacokinetics, supported by a pharmacokinetic simulation, demonstrate that the deep eutectic formulation reported here allows the maintenance of the therapeutic drug concentration in plasma in humans with a dosing regimen of approximately three injections per week compared to the current clinical practice of three injections per day.

Lipophilic Conjugates of Drugs: A Tool to Improve Drug Pharmacokinetic and Therapeutic Profiles

Lipophilic conjugates (LCs) of small molecule drugs have been used widely in clinical and pre-clinical studies to achieve a number of pharmacokinetic and therapeutic benefits. For example, lipophilic derivatives of drugs are employed in several long acting injectable products to provide sustained drug exposure for hormone replacement therapy and to treat conditions such as neuropsychiatric diseases. LCs can also be used to modulate drug metabolism, and to enhance drug permeation across membranes, either by increasing lipophilicity to enhance passive diffusion or by increasing protein-mediated active transport. Furthermore, such conjugation strategies have been employed to promote drug association with endogenous macromolecular carriers (e.g. albumin and lipoproteins), and this in turn results in altered drug distribution and pharmacokinetic profiles, where the changes can be 'general' (e.g. prolonged plasma half-life) or 'specific' (e.g. enhanced delivery to specific tissues in parallel with the macromolecular carriers). Another utility of LCs is to enhance the encapsulation of drugs within engineered nanoscale drug delivery systems, in order to best take advantage of the targeting and pharmacokinetic benefits of nanomedicines. The current review provides a summary of the mechanisms by which lipophilic conjugates, including in combination with delivery vehicles, can be used to control drug delivery, distribution and therapeutic profiles. The article is structured into sections which highlight a specific benefit of LCs and then demonstrate this benefit with case studies. The review attempts to provide a toolbox to assist researchers to design and optimise drug candidates, including consideration of drug-formulation compatibility.

Fail-safe nano-formulation of prodrug of sulfapyridine: Preparation and evaluation for treatment of rheumatoid arthritis

Aim of the present study was to give a second life to the long-abandoned drug, sulfapyridine (SP) for its anti-arthritic potential by design of nano-vesicular delivery system. For this, intra-articular delivery of its liposomal formulation was tried. As the prepared formulation exhibited rapid drug leakage, an arthritis responsive prodrug of SP showing lability towards synovial enzymes was synthesized to exploit the over-expression of arthritis specific enzymes. Prodrug (SP-PD) exhibited better retention in liposomes as compared to the drug, preventing its escape from synovium. Hydrolysis of SP-PD in human plasma and synovial fluid indicated its high susceptibility to enzymes. The liposomes of SP-PD exhibited larger mean size, less PDI and higher zeta potential as compared to those for SP liposomes. In arthritic rats, prodrug liposomes were found to reverse the symptoms of inflammation, including the levels of biochemical markers. Liposomes of bio-responsive prodrug, therefore, offer a revolutionary approach in the treatment of rheumatoid arthritis.

Structure-Functional-Selectivity Relationship Studies of Novel Apomorphine Analogs to Develop D1R/D2R Biased Ligands

Loss of dopamine neurons is central to the manifestation of Parkinson's disease motor symptoms. The dopamine precursor L-DOPA, the most commonly used therapeutic agent for Parkinson's disease, can restore normal movement yet cause side-effects such as dyskinesias upon prolonged administration. Dopamine D1 and D2 receptors activate G-protein- and arrestin-dependent signaling pathways that regulate various dopamine-dependent functions including locomotion. Studies have shown that shifting the balance of dopamine receptor signaling toward the arrestin pathway can be beneficial for inducing normal movement, while reducing dyskinesias. However, simultaneous activation of both D1 and D2Rs is required for robust locomotor activity. Thus, it is desirable to develop ligands targeting both D1 and D2Rs and their functional selectivity. Here, we report structure-functional-selectivity relationship (SFSR) studies of novel apomorphine analogs to identify structural motifs responsible for biased activity at both D1 and D2Rs.

Emerging Roles for Lymphatics in Chronic Liver Disease

Chronic liver disease (CLD) is a global health epidemic causing ∼2 million deaths annually worldwide. As the incidence of CLD is expected to rise over the next decade, understanding the cellular and molecular mediators of CLD is critical for developing novel therapeutics. Common characteristics of CLD include steatosis, inflammation, and cholesterol accumulation in the liver. While the lymphatic system in the liver has largely been overlooked, the liver lymphatics, as in other organs, are thought to play a critical role in maintaining normal hepatic function by assisting in the removal of protein, cholesterol, and immune infiltrate. Lymphatic growth, permeability, and/or hyperplasia in non-liver organs has been demonstrated to be caused by obesity or hypercholesterolemia in humans and animal models. While it is still unclear if changes in permeability occur in liver lymphatics, the lymphatics do expand in number and size in all disease etiologies tested. This is consistent with the lymphatic endothelial cells (LEC) upregulating proliferation specific genes, however, other transcriptional changes occur in liver LECs that are dependent on the inflammatory mediators that are specific to the disease etiology. Whether these changes induce lymphatic dysfunction or if they impact liver function has yet to be directly addressed. Here, we will review what is known about liver lymphatics in health and disease, what can be learned from recent work on the influence of obesity and hypercholesterolemia on the lymphatics in other organs, changes that occur in LECs in the liver during disease and outstanding questions in the field.

Anticancer activities of fatty acids and their heterocyclic derivatives

Traditional chemotherapy relies on the premise that rapidly proliferating cancer cells are more likely to be killed by a cytotoxic agent, but in reality, the long-standing problem of chemotherapy is the lack of tumor-specific treatments. Apart from the impact on tumor cells, the drugs' major limitation is their severe adverse side effects on normal cells and tissues. Nutritional and epidemiological studies have indicated that cancer progression is correlated with the consumption of fatty acids, but the exact mechanisms still remain unknown. In the first part of our review, we discussed the beneficial effects of free fatty acids (saturated and unsaturated) on the progress of carcinogenesis in different tumor cell lines. We presented various mechanisms proposed in the literature, which explain the possible impact on the cells metabolism. The second part describes modifications of different fatty acids with existing anticancer drugs and heterocyclic moieties by condensation reactions. Such conjugations increased the tissue selectivity and made chemotherapy potentially more effective and less toxic in in vivo and in vitro studies. This fatty acid modifications, which change the activity of compounds, their uptake selectivity and alter drug delivery methods, may be the key to unlocking true medical potential of fatty acids.

Formulating a single thioether-bridged oleate prodrug into a self-nanoemulsifying drug delivery system to facilitate oral absorption of docetaxel

Oral chemotherapy of docetaxel (DTX) is restricted by active P-glycoprotein (P-gp) efflux, hepatic first-pass metabolism and then poor oral absorption. Herein, a lipophilic thioether-bridged oleate prodrug (DTX-S-OA) and an ester-bond linked oleate prodrug of docetaxel (DTX-OA) were synthesized and efficiently incorporated into a self-nanoemulsifying drug delivery system (SNEDDS) using core-matching technology with a high drug-loading rate. DTX-S-OA SNEDDS produced a uniform droplet size of about 30 nm and a significantly high drug loading capability (60 mg mL-1), compared with DTX SNEDDS (20 mg mL-1). Additionally, DTX-S-OA SNEDDS exhibited a markedly slower drug release property and higher (>2-fold) drug solubilization in the aqueous phase after 60 min lipolysis compared with DTX SNEDDS. In situ single-pass intestinal perfusion and intestinal biodistribution studies demonstrated that the membrane permeability and intestinal bioadhesion of SNEDDS were significantly increased. Moreover, DTX-S-OA showed a comparable ability with verapamil in inhibiting P-gp efflux. Lymphatic transport studies confirmed that DTX-S-OA SNEDDS could significantly enhance intestinal lymphatic transport. Notably, the bioavailability of DTX-S-OA SNEDDS was 6.2-fold and 2.0-fold higher than that of the DTX solution and DTX SNEDDS, respectively. Furthermore, DTX-S-OA achieved a more rapid release of free DTX from the prodrug in systemic circulation than DTX-OA. Therefore, such a unique combination strategy of the single thioether-bridged DTX-oleate prodrug and SNEDDS is a promising platform to enable effective oral delivery of DTX.

Drug-Lipid Conjugates for Enhanced Oral Drug Delivery

Oral drug delivery route is one of the most convenient and extensively utilised routes for drug administration. But there exists class of drugs which exhibit poor bioavailability on oral drug administration. Designing of drug-lipid conjugates (DLCs) is one of the rationale strategy utilised in overcoming this challenge. This review extensively covers the various dimensions of drug modification using lipids to attain improved oral drug delivery. DLCs help in improving oral delivery by providing benefits like improved permeability, stability in gastric environment, higher drug loading in carriers, formation of self-assembled nanostructures, etc. The clinical effectiveness of DLCs is highlighted from available marketed drug products along with many DLCs in phase of clinical trials. Conclusively, this drug modification strategy can potentially help in augmenting oral drug delivery in future.

Synthesis, Biological Evaluation and Low-Toxic Formulation Development of Glycosylated Paclitaxel Prodrugs

Paclitaxel (PTX) is a famous anti-cancer drug with poor aqueous solubility. In clinical practices, Cremophor EL (polyethoxylated castor oil), a toxic surfactant, is used for dissolution of PTX, which accounts for serious side effects. In the present study, a single glucose-conjugated PTX prodrug (SG-PTX) and a double glucose-conjugated PTX prodrug (DG-PTX) were synthesized with a glycosylated strategy via succinate linkers. Both of the two prodrugs presented significant solubility improvement and drug-like lipophilicities. Compared to DG-PTX, SG-PTX manifested more promising release of the parent drug in serum. A high percentage of PTX released from SG-PTX could be detected after enzymatic hydrolysis of β-glucuronidase. Besides, both of the two prodrugs exhibited effective cytotoxicity against breast cancer cells and ovarian cancer cells, but presented reduced cytotoxicity against normal breast cells. Moreover, SG-PTX manifested impressive solubility in a low toxic formulation (without ethanol) with a different percentage of Cremophor EL. These results indicated that glycosylation is a promising strategy for PTX modification and SG-PTX may be a feasible and potential type of PTX prodrug. In addition, ethanol-free formulation with a low percentage of Cremophor EL might have the potential to develop a safer formulation for further studies of glycosylated PTX prodrugs.

Lipophilic activated ester prodrug approach for drug delivery to the intestinal lymphatic system

The intestinal lymphatic system plays an important role in the pathophysiology of multiple diseases including lymphomas, cancer metastasis, autoimmune diseases, and human immunodeficiency virus (HIV) infection. It is thus an important compartment for delivery of drugs in order to treat diseases associated with the lymphatic system. Lipophilic prodrug approaches have been used in the past to take advantage of the intestinal lymphatic transport processes to deliver drugs to the intestinal lymphatics. Most of the approaches previously adopted were based on very bulky prodrug moieties such as those mimicking triglycerides (TG). We now report a study in which a lipophilic prodrug approach was used to efficiently deliver bexarotene (BEX) and retinoic acid (RA) to the intestinal lymphatic system using activated ester prodrugs. A range of carboxylic ester prodrugs of BEX were designed and synthesised and all of the esters showed improved association with chylomicrons, which indicated an improved potential for delivery to the intestinal lymphatic system. The conversion rate of the prodrugs to BEX was the main determinant in delivery of BEX to the intestinal lymphatics, and activated ester prodrugs were prepared to enhance the conversion rate. As a result, an 4-(hydroxymethyl)-1,3-dioxol-2-one ester prodrug of BEX was able to increase the exposure of the mesenteric lymph nodes (MLNs) to BEX 17-fold compared to when BEX itself was administered. The activated ester prodrug approach was also applied to another drug, RA, where the exposure of the MLNs was increased 2.4-fold through the application of a similar cyclic activated prodrug. Synergism between BEX and RA was also demonstrated in vitro by cell growth inhibition assays using lymphoma cell lines. In conclusion, the activated ester prodrug approach results in efficient delivery of drugs to the intestinal lymphatic system, which could benefit patients affected by a large number of pathological conditions.

Prodrugs, phospholipids and vesicular delivery - An effective triumvirate of pharmacosomes

With the advent from the laboratory bench to patient bedside in last five decades, vesicular systems have now come to be widely accepted as pragmatic means for controlled delivery of drugs. Their success stories include those of liposomes, niosomes and even the lately developed ethosomes and transferosomes. Pharmacosomes, which, as delivery systems offer numerous advantages and have been widely researched, however, remain largely unacknowledged as a successful delivery system. Though a large number of drugs have been derivatized and formulated into self-assembled vesicular systems, the term pharmacosomes has not been widely used while reporting them. Therefore, their relative obscurity may be attributed to the non-usage of the nomenclature of pharmacosomes by the researchers working in the area. We present a review on the scenario that lead to origin of these bio-inspired vesicles composed of self-assembling amphiphilic molecules. Various drugs that have been formulated into pharmacosomes, their characterization techniques, their properties relative to those of other vesicular delivery systems, and the success achieved so far are also discussed.

A lipophilic prodrug of Danshensu: preparation, characterization, and in vitro and in vivo evaluation

Danshensu [3-(3, 4-dihydroxyphenyl) lactic acid, DSS], one of the significant cardioprotective components, is extracted from the root of Salvia miltiorrhiza. In the present study, an ester prodrug of Danshensu (DSS), palmitoyl Danshensu (PDSS), was synthesized with the aim to improve its oral bioavailability and prolong its half-life. The in vitro experiments were carried out to evaluate the physicochemical properties and stability of PDSS. Although the solubility of PDSS in water was only 0.055 mg·mL^-1, its solubility in FaSSIF and FeSSIF reached 4.68 and 9.08 mg·mL^-1, respectively. Octanol-water partition coefficient (log P) was increased from -2.48 of DSS to 1.90 of PDSS. PDSS was relatively stable in the aqueous solution in pH range from 5.6 to 7.4. Furthermore, the pharmacokinetics in rats was evaluated after oral administration of PDSS and DSS. AUC and t_1/2 of PDSS were enhanced up to 9.8-fold and 2.2-fold, respectively, compared to that of DSS. C_max was 1.67 ± 0.11 μg·mL^-1 for PDSS and 0.81 ± 0.06 μg·mL^-1 for DSS. Thus, these results demonstrated that PDSS had much higher oral bioavailability and longer circulation time than its parent drug.

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.

Lipid-Drug Conjugate for Enhancing Drug Delivery

Lipid-drug conjugates (LDCs) are drug molecules that have been covalently modified with lipids. The conjugation of lipids to drug molecules increases lipophilicity and also changes other properties of drugs. The conjugates demonstrate several advantages including improved oral bioavailability, improved targeting to the lymphatic system, enhanced tumor targeting, and reduced toxicity. Based on the chemical nature of drugs and lipids, various conjugation strategies and chemical linkers can be utilized to synthesize LDCs. Linkers and/or conjugation methods determine how drugs are released from LDCs and are critical for the optimal performance of LDCs. In this review, different lipids used for preparing LDCs and various conjugation strategies are summarized. Although LDCs can be administered without a delivery carrier, most of them are loaded into appropriate delivery systems. The lipid moiety in the conjugates can significantly enhance drug loading into hydrophobic components of delivery carriers and thus generate formulations with high drug loading and superior stability. Different delivery carriers such as emulsions, liposomes, micelles, lipid nanoparticles, and polymer nanoparticles are also discussed in this review.

50years of oral lipid-based formulations: Provenance, progress and future perspectives

Lipid based formulations (LBF) provide well proven opportunities to enhance the oral absorption of drugs and drug candidates that sit close to, or beyond, the boundaries of Lipinski's 'rule-of-five' chemical space. Advantages in permeability, efflux and presystemic metabolism are evident; however, the primary benefit is in increases in dissolution and apparent intestinal solubility for lipophilic, poorly water soluble drugs. This review firstly details the inherent advantages of LBF, their general properties and classification, and provides a brief retrospective assessment of the development of LBF over the past fifty years. More detailed analysis of the ability of LBF to promote intestinal solubilisation, supersaturation and absorption is then provided alongside review of the methods employed to assess formulation performance. Critical review of the ability of simple dispersion and more complex in vitro digestion methods to predict formulation performance subsequently reveals marked differences in the correlative ability of in vitro tests, depending on the properties of the drug involved. Notably, for highly permeable low melting drugs e.g. fenofibrate, LBF appear to provide significant benefit in all cases, and sustained ongoing solubilisation may not be required. In other cases, and particularly for higher melting point drugs such as danazol, where re-dissolution of crystalline precipitate drug is likely to be slow, correlations with ongoing solubilisation and supersaturation are more evident. In spite of their potential benefits, one limitation to broader use of LBF is low drug solubility in the excipients employed to generate formulations. Techniques to increase drug lipophilicity and lipid solubility are therefore explored, and in particular those methods that provide for temporary enhancement including lipophilic ionic liquid and prodrug technologies. The transient nature of these lipophilicity increases enhances lipid solubility and LBF viability, but precludes enduring effects on receptor promiscuity and off target toxicity. Finally, recent efforts to generate solid LBF are briefly described as a means to circumvent the need to encapsulate in soft or hard gelatin capsules, although the latter remain popular with consumers and a proven means of LBF delivery.

Prodrugs for transdermal drug delivery - trends and challenges

Prodrugs continue to attract significant interest in the transdermal drug delivery field. These moieties can confer favorable physicochemical properties on transdermal drug delivery candidates. Alkyl chain lengthening, pegylation are some of the strategies used for prodrug synthesis. It is usually important to optimize partition coefficient, water and oil solubilities of drugs. In this review, progress made in the field of prodrugs for percutaneous penetration is highlighted and the challenges discussed.