Vol. 27, 2018 Drug Discovery Today: Technologies Editors-in-Chief Kelvin Lam – Simplex Pharma Advisors, Inc., Boston, MA, USA Henk Timmerman – Vrije Universiteit, The Netherlands DRUG DISCOVERY TODAY TECHNOLOGIES Physicochemical characterisation in drug discovery The influence and manipulation of acid/ base properties in drug discovery David T. Manallack*, Elizabeth Yuriev, David K. Chalmers Faculty of Pharmacy and Pharmaceutical Sciences, Monash Institute of Pharmaceutical Sciences, Monash University, 381 Royal Parade, Parkville, VIC 3052, Australia There is a growing awareness of the importance of acid/ Section editor: base properties inmedicinalchemistryresearch.Inmany Dr. György T. Balogh – Head of laboratory, honorary fi drug classes, ionisable groups are present that make associate professor, Compound Pro ling Laboratory, Gedeon Richter Plc. critical interactions with the receptor and are essential for potency. Yet the presence of these groups may cause problems with oral bioavailability, pharmacokinetics, or arises from the ionisation of weak to moderately-strong acids toxicity. Manipulating pKa values during drug develop- and bases present in the protein and drug. For example, in ment or applying pro-drug techniques are strategies that biogenic amine G protein-coupled receptors (GPCRs), such as can overcome potential deficits in a variety of these the dopamine or serotonin receptors, an ionisable amino group in the ligand forms a strong ionic interaction (a areas. Knowledge of drug ionisation states coupled with ‘‘salt-bridge’’) with a negatively charged aspartic acid residue a consideration of pH-specific cellular environments can within the binding site [1]. Removal of either the positive be used advantageously to target chemoresistance. As charge from the ligand or the negatively charged residue from modern drug research ventures into drug candidates the protein generally abolishes biological activity. Other drug classes, for example the ‘‘statin’’ inhibitors of cholesterol that exceed the rule of 5, such exploration requires an biosynthesis, require an acidic functional group on the ligand understanding of drug acid/base properties and how in order to have sufficient potency. The carboxylate group of these factors affect ADMET characteristics. these drugs forms a strong ionic interaction with a charged lysine and an adjacent serine residue of HMG-CoA reductase, replicating a similar interaction made by the substrate, HMG- CoA [2]. Removal of the carboxylate greatly reduces potency. Introduction The strength of the interaction between ionised acid/base The attraction or repulsion between electrostatic charges pairs has important implications for drug development. In underlies many chemical phenomena. Ionic interactions, many drug targets, the contribution of the ionic interaction hydrogen bonding and dispersion forces, are fundamentally to ligand affinity makes it difficult (or impossible) for the charge–charge interactions, and the interaction between a medicinal chemistto substitutethe ionisable functional group, drug and its protein binding site is dictated by the comple- leading to these groups being a ‘non-negotiable’ feature of mentarity of drug and binding site charge and shape. Fre- many drug class chemotypes(Fig. 1). Recentreviews have given quently, the binding of a drug to a protein target is dominated insights into the acid/base profiles of drugs [3,4], biologically by a single strong electrostatic interaction that, in most cases, active substances, and screening compounds [5]. Importantly, *Corresponding author: D.T. Manallack ([email protected]) the presence of ionised functional groups within drugs broadly 1740-6749/$ © 2018 Elsevier Ltd. All rights reserved. https://doi.org/10.1016/j.ddtec.2018.04.003 41 Drug Discovery Today: Technologies | Physicochemical characterisation in drug discovery Vol. 27, 2018 Risperidone Atorvastatin Vorioxetine Salbutamol Statins Anti- GPCRs Naproxen depressants NSAIDs Diclofenac Escitalopram Amines COOH Anti- Steroids Neutral SO2NHR Acid microbials Fluticasone propionate Base Sulfamethoxazole Phosphorus N-containing containing acids heterocycles (acid) Anti- (base) epileptics Osteoporosis Anti- nausea Alendronate sodium Ondansetron Phenobarbitone Drug Discovery Today: Technologies Fig. 1. Examples of compound or therapeutic drug classes and associated ionisable functional groups. ``Neutral” refers to their molecular state under physiological conditions. Passage across cells [permeability, absorption and distribu- impacts drug behaviour within the body. Acidic and basic tion] characteristics, in combination with whole molecule lipophi- Toxicity [e.g. hERG channel blockade, phospholipidosis] licity, affect drug behaviour in a number of broad areas: Pharmacokinetics [clearance, plasma and tissue binding, Molecular interactions with the target macromolecule metabolism] 42 www.drugdiscoverytoday.com Vol. 27, 2018 Drug Discovery Today: Technologies | Physicochemical characterisation in drug discovery Formulation [dissolution within the GI tract, stability], and However, in an increasing number of cases, alternative com- Environmental impact pound classes or binding modes can be identified that can In addition to the important role of ionisable groups in remove the requirement for the acid or base to be present. A forming drug-receptor interactions, drug acidic and basic case in point is found with non-steroidal anti-inflammatory groups impinge on many aspects of drug behaviour; quite drugs (NSAIDs). NSAIDs bind to the cyclooxygenase (COX) often,unfavourably. Charged,polargroupscanadverselyaffect enzyme, which performs a key step in the inflammation drug solubility, oral absorption and distribution, protein plas- pathway. While all early NSAIDs contained a carboxylic acid ma binding, and can introduce toxicity through off-target group that was thought to make an essential interaction with effects such as interactions with the hERG ion channel. To Arg 120 in the COX catalytic site (e.g. ibuprofen, Fig. 2), address these issues, there is often a need to modulate the acid/ research into alternative chemotypes began in the 1960s. At base behaviour of the candidate compounds during the drug the time, NSAID drugs that contained a carboxylic acid development process. A variety of approaches are possible; needed multiple daily doses due to rapid metabolism and there is scope to use pro-drugs, manipulate the pKa values excretion. Stringent design criteria were used to find agents through structural changes, utilise bioisosteres with improved that could be dosed once a day. From this research the oxicam properties, exploit pH microenvironments for better drug tar- class of NSAIDs emerged in the late 1970s, where Pfizer geting, or take advantage of active transport mechanisms. In specifically sought to find potent COX inhibitors without a larger compounds, ionisable groups generally have detrimen- carboxylic acid group (e.g. piroxicam, Fig. 2). The oxicam tal effects on drug properties, for example causing low perme- class of compounds still contains the weakly acidic enolate ability. This mini-review explores some of the above issues, group (Fig. 2) and binds to the same location as other NSAIDs providing medicinal chemistry examples with additional com- but the enolate does not interact directly with Arg 120 [6]. K mentary on less druggable targets. While piroxicam also contains a weak pyridine base (p a 2.33), this particular group would only be ionised at very Strategies to deal with problematic acidic or basic low pH values. The development of selective COX-2 inhibi- functional groups in small molecules tors exposed further structural classes that lack acidic func- As discussed above, strong ionic interactions are empirically tionalities (e.g. celecoxib, Fig. 2). This illustration serves to essential and non-removable features of many drug classes. demonstrate that ionisable groups can be replaced but success Ibuprofen Piroxicam Celecoxib Compound 15 Drug Discovery Today: Technologies Fig. 2. Inhibitors of cyclooxygenase, a key enzyme in inflammation pathways: ibuprofen (pKa 4.9) is ionised at physiological pH; piroxicam and celecoxib are weaker acids (pKa values 5.1 and 10.7, respectively). The enolate group of piroxicam is highlighted with a dashed red box. Compound 15 (Liu et al. [7]) binds to an allosteric site on the b2 adrenergic receptor and functions as an antagonist. www.drugdiscoverytoday.com 43 Drug Discovery Today: Technologies | Physicochemical characterisation in drug discovery Vol. 27, 2018 often necessitates a large body of knowledge about the bind- idine analogues, or a piperazine group was used to reduce ing requirements of the target to be gathered. hERG affinity [11]. The lower pKa values of basic compounds Alternatively, the requirement for a charged group in the also increased oral absorption without affecting potency primary binding site can be circumvented by identifying significantly [11]. In a study of anti-malarial drugs, Dong K alternative, functional binding pockets or allosteric sites. et al. [12] found that higher p a values were associated with In GPCRs that recognise biogenic amines, designing drugs higher metabolic stability. While high basicity is usually for orthosteric sites can pose selectivity problems and most regarded as undesirable, in this case, metabolic stability receptors have an absolute requirement for a basic amino was a fundamental need for single dose anti-malarial treat- group. A recent crystal structure of the b2 adrenergic