The Medicinal Chemist’s Toolbox: An Analysis of Reactions Used in the Pursuit of Drug Candidates

The Medicinal Chemist's Toolbox: An Analysis of Reactions Used in the Pursuit of Drug Candidates

Prodrugs are bioreversible derivatives of drug molecules that undergo an enzymatic and/or chemical transformation in vivo to release the active parent drug, which can then exert the desired pharmacological effect. In both drug discovery and development, prodrugs have become an established tool for improving physicochemical, biopharmaceutical or pharmacokinetic properties of pharmacologically active agents. About 5-7% of drugs approved worldwide can be classified as prodrugs, and the implementation of a prodrug approach in the early stages of drug discovery is a growing trend. To illustrate the applicability of the prodrug strategy, this article describes the most common functional groups that are amenable to prodrug design, and highlights examples of prodrugs that are either launched or are undergoing human trials.

Prodrugs: design and clinical applications

Drugs with low water solubility are predisposed to low and variable oral bioavailability and, therefore, to variability in clinical response. Despite significant efforts to "design in" acceptable developability properties (including aqueous solubility) during lead optimization, approximately 40% of currently marketed compounds and most current drug development candidates remain poorly water-soluble. The fact that so many drug candidates of this type are advanced into development and clinical assessment is testament to an increasingly sophisticated understanding of the approaches that can be taken to promote apparent solubility in the gastrointestinal tract and to support drug exposure after oral administration. Here we provide a detailed commentary on the major challenges to the progression of a poorly water-soluble lead or development candidate and review the approaches and strategies that can be taken to facilitate compound progression. In particular, we address the fundamental principles that underpin the use of strategies, including pH adjustment and salt-form selection, polymorphs, cocrystals, cosolvents, surfactants, cyclodextrins, particle size reduction, amorphous solid dispersions, and lipid-based formulations. In each case, the theoretical basis for utility is described along with a detailed review of recent advances in the field. The article provides an integrated and contemporary discussion of current approaches to solubility and dissolution enhancement but has been deliberately structured as a series of stand-alone sections to allow also directed access to a specific technology (e.g., solid dispersions, lipid-based formulations, or salt forms) where required.

Strategies to Address Low Drug Solubility in Discovery and Development

Ion exchange membranes: State of their development and perspective

Ionic liquids are remarkable chemical compounds, which find applications in many areas of modern science. Because of their highly tunable nature and exceptional properties, ionic liquids have become essential players in the fields of synthesis and catalysis, extraction, electrochemistry, analytics, biotechnology, etc. Apart from physical and chemical features of ionic liquids, their high biological activity has been attracting significant attention from biochemists, ecologists, and medical scientists. This Review is dedicated to biological activities of ionic liquids, with a special emphasis on their potential employment in pharmaceutics and medicine. The accumulated data on the biological activity of ionic liquids, including their antimicrobial and cytotoxic properties, are discussed in view of possible applications in drug synthesis and drug delivery systems. Dedicated attention is given to a novel active pharmaceutical ingredient-ionic liquid (API-IL) concept, which suggests using traditional drugs in ...

Biological Activity of Ionic Liquids and Their Application in Pharmaceutics and Medicine.

The blood-brain barrier efficiently controls the entry of drug molecules into the brain. We describe a feasible means to achieve carrier-mediated drug transport into the rat brain via the specific, large neutral amino acid transporter (LAT1) by conjugating a model compound to L-tyrosine. A hydrophilic drug, ketoprofen, that is not a substrate for LAT1 was chosen as a model compound. The mechanism and the kinetics of the brain uptake of the prodrug were determined with an in situ rat brain perfusion technique. The brain uptake of the prodrug was found to be concentration-dependent. In addition, a specific LAT1 inhibitor significantly decreased the brain uptake of the prodrug. Therefore, our results reveal for the first time that a drug-substrate conjugate is able to transport drugs into the brain via LAT1.

Large Neutral Amino Acid Transporter Enables Brain Drug Delivery via Prodrugs

Central nervous system (CNS) drug delivery remains a major challenge, despite extensive efforts that have been made to develop novel strategies to overcome obstacles. Prodrugs are bioreversible derivatives of drug molecules that must undergo an enzymatic and/or chemical transformation in vivo to release the active parent drug, which subsequently exerts the desired pharmacological effect. In both drug discovery and drug development, prodrugs have become an established tool for improving physicochemical, biopharmaceutical or pharmacokinetic properties of pharmacologically active agents that overcome barriers to a drug’s usefulness. This review provides insight into various prodrug strategies explored to date for CNS drug delivery, including lipophilic prodrugs, carrier- and receptor-mediated prodrug delivery systems, and gene-directed enzyme prodrug therapy.

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Prodrug approaches for CNS delivery.

INTRODUCTION. The main action of paracetamol (acetaminophen) is presumed to be in the central nervous system. The central nervous system penetration of paracetamol has been described in children with intracranial pathologies but not in children with an intact blood-brain barrier. OBJECTIVE. We investigated the cerebrospinal fluid penetration of paracetamol in 32 healthy children, aged 3 months to 12 years, who were undergoing surgery in the lower body using spinal anesthesia. MATERIALS AND METHODS. In this open-label prospective study, children were given a single intravenous injection of paracetamol (15 mg/kg). Cerebrospinal fluid and venous blood samples were obtained between 5 minutes and 5 hours after injection. Paracetamol concentrations were determined from the cerebrospinal fluid and plasma by using a fluorescence polarization immunoassay. RESULTS. Paracetamol was detected in cerebrospinal fluid from the earliest sample at 5 minutes, although in this sample paracetamol concentration was below the limit of quantification of 1.0 mg/L. Subsequent paracetamol concentrations in cerebrospinal fluid ranged between 1.3 and 18 mg/L (median: 7.2 mg/L), plasma concentrations ranged between 2.4 and 33 mg/L, and cerebrospinal fluid/plasma ratios ranged between 0.06 and 2.0. The highest CSF paracetamol concentration was detected at 57 minutes. CONCLUSIONS. Paracetamol permeates readily into the cerebrospinal fluid of children. This fast and extensive transfer enables the rapid central analgesic and antipyretic action of intravenous paracetamol.

Paracetamol (acetaminophen) penetrates readily into the cerebrospinal fluid of children after intravenous administration

The aim of this study was to investigate the effect of cyclodextrins (s-CD, HP-s-CD and (SBE)7m-s-CD), and coadministration of water-soluble polymer (hydroxypropyl-methylcellulose; HPMC) and cyclodextrins (CDs) on aqueous solubility and oral bioavailability of glibenclamide (GBA). GBA was administered orally and intravenously to dogs. GBA showed AL-type diagram for s-CD and Ap-type diagrams for both s-CD derivatives studied. HPMC (0.05 %) enhanced the solubilizing effect of CDs but did not affect the type of phase-solubility diagram. Orally administered GBA and its physical mixture with HP-s-CD showed poor absolute bioavailability while GBA/s-CD, GBA/HP-s-CD and GBA/(SBE)7m-s-CD enhanced significantly the absolute bioavailability of GBA. Orally administered GBA/s-CD/HPMC and GBA/(SBE)7m-s-CD/HPMC showed similar absolute bioavailability compared to formulations not containing HPMC even though 80 % (in the case of (SBE)7m-s-CD) or 40 % (in the case of s-CD) less CD was used. In conclusion, the oral bioavailability of GBA was significantly increased by CD-complexation and the amount of CD needed was significantly reduced by co-administration of HPMC.

Jouko Savolainen

Papers

Prodrugs: design and clinical applications

Large Neutral Amino Acid Transporter Enables Brain Drug Delivery via Prodrugs

Prodrug approaches for CNS delivery.

Paracetamol (acetaminophen) penetrates readily into the cerebrospinal fluid of children after intravenous administration

Co-administration of a water-soluble polymer increases the usefulness of cyclodextrins in solid oral dosage forms.