The strategy of chemical modification to reduce the cardiotoxicity of drug hERG

Cardiac safety assessment assays can be used for pre-clinical assessment of compound safety to identify toxic drugs that may cause fatal cardiac tachyarrhythmias.

Ion channels are a diverse class of membrane proteins that play important roles in physiology. Insufficient or increased ion channel activity caused by pharmacological activity has been associated with several disease conditions. Inhibition of cardiac ion channels is associated with the development of life-threatening arrhythmias. Therefore, it is important to screen compounds against these ion channels to assess potential cardiac function.

Drugs that inhibit hERG channel activity are routinely screened out, as this implies a fatal QT prolongation side effect.

Reduce fat solubility

Quantitative structure-activity relationship (QSAR) is the establishment of quantitative or qualitative dependencies between the chemical structure of molecules and their biological effects. It integrates physical chemistry, physical organic chemistry, quantum chemistry, biochemistry, pharmacology, statistics, and computational science. knowledge and methods.

Through QSAR analysis, Levoin et al. pointed out that the lipid solubility (clogP, clogD or polar surface area PSA) and aromaticity of the molecule are closely related to the hERG inhibitory activity. The fat-soluble aromatic ring in the drug molecule has a π-π hydrophobic interaction with the hERG potassium channel. Reducing the lipid solubility of the molecule, such as introducing electron withdrawing groups or polar groups on the aromatic ring of the drug molecule, or replacing the benzene ring with a heterocyclic ring through an isosteric, can effectively hinder the hydrophobic effect and reduce hERG inhibition active.

Adenosine receptor A2A antagonists can be used to treat Parkinson’s. Compound 1 is a lead compound of A2A antagonists reported by Merck (IC50 = 5.5 nmol·L-1), which has good selectivity for adenosine receptor A1. But it has strong hERG inhibitory activity (IC50=1.5 μmol·L-1). Using the strategy of reducing the fat solubility, the terminal benzene ring was replaced with pyrazole to obtain compounds 1a and 1b. The fat-soluble clogP decreased by 1.9 and 0.7 units, respectively, and the hERG inhibitory activity was significantly decreased (IC50>60 μmol·L-1), while maintaining The activity of the compound on A2A and the selectivity for A1 are described (Table 2).

Compound 2 is a broad-spectrum antibacterial drug with good antibacterial activity. It has a strong inhibitory effect on topoisomerase IV. The IC50 of hERG inhibitory activity is 3 μmol·L-1. Substituting the azaquinoline isostere with the more polar quinolone to obtain compounds 2a～2c, the fat-soluble logD7.4 decreased by 0.6 to 1.6 units, and the hERG inhibitory activity decreased significantly (IC50> 30μmol·L-1) (Table 3) ).

Increasing the polar surface area (PSA) and reducing clogP can work synergistically to reduce fat solubility and improve hERG inhibition. Histamine H3 receptor antagonist 3, IC50 reaches 2.4～3.5 nmol·L-1, but hERG inhibitory activity is strong (IC50 = 1.1 μmol·L-1), the terminal benzene ring is replaced with dimethyl substituted oxazole Cyclic, clogP and clogD decreased by 0.3-0.4 units. At the same time, the dimethyl substitution on the oxazole ring nearly doubled the PSA, the lipid solubility of compound 3a decreased, and the inhibitory activity of its hERG potassium channel was significantly reduced (IC50= 37 μmol· L-1), at the same time the H3 receptor antagonistic activity is increased (IC50 = 0.1～0.8nmol·L-1) (Table 4).

Replacing the benzene ring with an aliphatic heterocyclic ring containing nitrogen or oxygen, such as piperidine, piperazine, tetrahydropyran, etc., can effectively reduce the lipid solubility (clopP) of the drug molecule and hinder the hydrophobic interaction between the drug molecule and the hERG potassium channel effect.

Compound 4 is a CCR5 antagonist (IC50 = 0.32 nmol·L-1) developed by AstraZeneca, and the inhibitory activity of the hERG potassium channel is 7.3 μmol·L-1. At a dose of 50 mg·kg-1 orally administered, compound 4 caused a prolonged QT interval in a canine model. Using a structural modification strategy to reduce fat solubility, replacing the benzene ring with piperidine or piperazine, logD7.4 decreased by 0.7-1.0 units, hERG inhibitory activity was greatly reduced (IC50=24 μmol·L-1), and CCR5 antagonistic activity Keep (Table 5).

A similar strategy has also been successfully applied to the structural optimization of H3 receptor antagonist 5. The antagonistic activity of compound 5 on H3 receptor is 1.2～16.5 nmol·L-1, and the hERG inhibitory activity is strong (IC50=0.48 μmol·L-1). The benzene ring was replaced with a tetrahydropyran ring, clogP decreased by 2.3 units, hERG inhibitory activity decreased to 1/39 of the original (IC50=18.9 μmol·L-1), and the antagonistic activity of compound 5a on H3 receptors was increased. (IC50 is 0.8～1.0 nmol·L-1) (Table 6).

Reduce alkalinity

Reducing basicity is an important strategy to optimize the structure of lead compounds to reduce hERG inhibitory activity. Some drug molecules are relatively alkaline and can be protonated under physiological conditions. Homologous modeling and site-directed mutagenesis experiments have shown that they can form strong π-cation interactions with the amino acid residue Tyr652 in the potassium channel of hERG.

Reducing the basicity (pKa) of the drug can hinder the π-cation interaction and reduce the inhibitory activity of hERG. Lowering basicity (pKa) includes: introducing electron withdrawing groups (such as introducing F, sulfonyl, heteroatom, carbonyl, amide, etc.); or replacing amino with amide, sulfonamide, etc.

When the pKa is reduced by introducing polar fragments (such as carbonyl and oxygen heterocycles, etc.), it will also lead to a decrease in fat solubility (logD). Therefore, reducing alkalinity and reducing fat solubility are interrelated and interact. However, sometimes reducing the alkalinity of the amino groups in the drug molecule will affect the activity and physical and chemical properties, because some amino groups have key hydrogen bond interactions with the target protein, while others are used as salt formation sites to improve the dissolution of the drug. This is some of the limitations of this method, which should be paid attention to in practical applications.

Broad-spectrum antibacterial drug compound 6 has good inhibitory activity on topoisomerase IV (IC50= 3.2 nmol·L-1), hERG inhibitory activity IC50 is 44 μmol·L-1, but it can cause QT in the Guinea pig model The interval is extended. The introduction of electrically-absorbing fluorine atoms into the piperidine ring reduced the pKa by 1.3 units, and the hERG inhibitory activity was reduced to 1/5 (IC50=233μmol·L-1), while the antibacterial activity remained unchanged.

Compound 7 is a cyclohexylamine DDP-IV inhibitor (IC50 reaches 0.5 nmol·L-1). It has a strong hERG inhibitory activity (IC50=4.8μmol·L-1), which can lead to a prolonged QT interval in a dog model. By introducing an oxygen atom at the β position of the basic amino group on the cyclohexyl ring, the pKa decreased from 8.6 to 7.3, and the hERG inhibitory activity was greatly reduced (IC50=23 μmol·L-1), and the inhibitory activity of DPP-IV was not affected.

Compound 8 is a 2-cyanopyrimidine cathepsin K (CatK) inhibitor developed by Merck. The hERG inhibitory activity IC50 is 0.16 μmol·L-1. The piperidine in the structure was replaced with an aminoamide fragment, the basic (pKa) and fat-soluble (clogP) were reduced by 1.8 and 2.2 units, respectively, and the hERG inhibitory activity was reduced to 1/30 of the original (IC50 = 3.16 μmol·L-1) , Rats were given compound 8a 100 mg·kg-1 for two weeks, and no adverse cardiac reactions were observed.

Replacing amino groups with amides can significantly reduce alkalinity and improve the inhibitory activity of hERG potassium channels. Compounds 9 and 10 are a class of tertiary amine T-type calcium channel blockers reported by Shin et al., and are potential lead compounds for the treatment of cardiovascular diseases. The hERG inhibitory activity is strong, with 0.18 and 1.38 μmol, respectively · L-1. Replacement of amino groups with amides reduced both alkalinity and fat solubility. The IC50 of hERG inhibitory activity was reduced to 12.5 and 16.8 μmol·L−1, respectively, but this method resulted in a slight decrease in calcium channel blocking activity .

Introducing hydroxyl

The hydroxyl group is a strong polar hydrogen bond donor group. The introduction of hydroxyl group can significantly change the physical and chemical properties of the molecule, reduce the fat solubility and alkalinity, and hinder the hydrophobic interaction and π-cation interaction between the drug molecule and the hERG potassium channel. In recent years, more and more research examples have proved that the introduction of hydroxyl groups plays an important role in improving the inhibitory activity of hERG.

Compound 11 is a melanin concentrating hormone receptor (MCHR) antagonist (IC50 of 13 nmol·L-1), but its hERG inhibitory activity is extremely strong (IC50=0.002 μmol·L-1). Using the above strategy to reduce fat solubility, replacing the benzene ring with a tetrahydropyran ring, the inhibitory activity of hERG decreased to 1/60 of 11 (IC50=0.12 μmol·L-1), and the inhibitory activity was still strong.

By introducing a hydroxyl group on the tetrahydropyran ring, the hERG inhibitory activity further decreased to 11 of 1/4 000 (IC50=8.24 μmol·L-1), while the MCHR antagonistic activity was maintained. It can be seen that the hydroxyl group plays an important role in improving the hERG inhibitory activity.

N-type calcium channel (NVDCC) is a potential target for the treatment of nerve pain, compound 12 is an effective N-type calcium channel blocker reported by Ogiyama et al. (IC50=0.6 nmol·L-1), hERG inhibition Strong activity (IC50=8.3 μmol·L-1). Using a similar structure optimization strategy, replacing the 2-methoxy-substituted benzene ring with a hydroxyl-substituted cyclohexyl fragment, the hERG activity decreased to 98 μmol·L-1.

Spindle protein (KSP) inhibitors can be used to fight tumors. Compound 13 is a KSP inhibitor (IC50 = 4nmol·L-1) developed by Merck, and has strong hERG inhibitory activity (IC50=1.2 μmol·L-1). Introducing a hydroxymethyl group at the 2-position of dihydropyrrole to obtain compound 13a, logP decreased from 2.5 to 1.7, hERG inhibitory activity decreased to 1/12 of the original (IC50=14.6 μmol·L-1), but compound 13a was multi-drug resistant The sex ratio (MDR) is high, and it is easy to be effluxed by the cell. By adjusting the side chain to introduce fluorine atoms in aminopiperidine, the pKa decreases by 1.2 to 2.2 units, and the hERG inhibitory activity further decreases (IC50>20.5 μmol·L-1). At the same time, the MDR dropped below 10 and cell viability increased.

Compound 14 is a selective cathepsin S (CatS) inhibitor (IC50=3 nmol·L-1, CatL/CatS=120). CatS is a type of cysteine protease, which is mainly expressed on dendritic cells, B cells, macrophages and brain microglia. It is responsible for protein hydrolysis and antigen presentation. CatS selectively inhibits spinal microglia. Reverse neuropathic pain. The ability of compound 14 to cross the blood-brain barrier is better, but the hERG inhibitory activity is stronger (IC50=0.71 μmol·L-1). The introduction of a hydroxyl group on the amino side chain has little effect on the hERG inhibitory activity.

By adopting the above-mentioned strategy of reducing alkalinity, the inhibitory activity of hERG reached IC50 greater than 30 μmol·L-1, but the ability of compound 14b to cross the blood-brain barrier decreased, and the drug concentration in the brain was less than 13 nmol·L-1. By replacing the amino side chain with hydroxyethyl, the inhibitory activity of hERG was greater than 30 μmol·L-1, and the activity, selectivity and blood-brain barrier ability of compound 14c on Cat were maintained.

Introducing acidic groups

The ingenious introduction of acidic fragments into drug molecules can form internal salts with basic amino groups, which can significantly reduce the lipid solubility of the molecule, reduce the basicity of the molecule, and reduce its interaction with the more hydrophobic hERG potassium channel; at the same time, it reduces The membrane permeability of the compound makes it difficult to pass through the filter area of the hERG potassium channel.

The introduction of acidic fragments is a direct and effective structural modification strategy to block the interaction between small molecule ligands and hERG channels. However, sometimes the introduction of acidic groups has a greater impact on the physical and chemical properties of drug molecules, and has a greater impact on the pharmacodynamics and pharmacokinetics of drug molecules. The kinetic properties have a greater impact.

Compound 15 is a highly effective and selective melanin aggregation hormone receptor MCHR1 antagonist (Ki=0.3 nmol·L-1, IC50=0.5 nmol·L-1) developed by Amgen, which is used for obesity treatment. Compound 15 has a strong inhibitory effect on the hERG potassium channel (IC50 = 0.03 μmol·L-1).

The introduction of a carboxylic acid group at the 4-position of tetrahydropyran yields compound 15a, and its hERG inhibitory activity is reduced to 1/10 (IC50 = 0.3 μmol·L-1), but the MCHR1 antagonistic activity is significantly reduced, and the compound is shortened after the carbon chain The antagonistic activity of MCHR of 15b and 15c was increased.

Substituting tetrahydropyran with cyclohexyl to obtain compound 15c, MCHR activity was maintained (IC50 = 0.6 nmol·L-1), and hERG inhibitory activity decreased to 1/166 of that of compound 15 (IC50>5 μmol·L-1) ), showed good pharmacokinetic properties in rats, dogs, monkeys and other models, and no adverse effects of prolonged QT interval were observed. In addition, compound 15c has a strong ability to penetrate the blood-brain barrier. It is effective in animal models such as dogs and monkeys.

Conformation restrictions

Make subtle adjustments to the basic skeleton of drug molecules, such as changing the chirality, introducing methyl groups, ring expansion, or introducing double bonds to increase molecular rigidity, limit drug molecular conformation or reduce the number of flexible conformations, which can effectively hinder drug molecules and hERG potassium Channel (off-target) interaction; at the same time, because the pharmacophore remains unchanged, this strategy has little effect on the efficacy. In recent years, there have been more and more reports on improving the inhibitory activity of hERG through conformational restriction, providing new ideas for improving the inhibitory activity of drugs hERG.

1 Changing the chirality

Chiral drugs are becoming more and more common in the marketed drugs, and different chirality often leads to different properties such as drug pharmacodynamics, pharmacokinetics and toxicology. Similarly, the different isomers of chiral drug molecules have different inhibitory effects on hERG potassium channels. For example, compounds (R, R)-16 and (R, R)-17 are farnesyltransferase inhibitors, which have Good activity (IC50<1 nmol·L-1), hERG inhibitory activity is 0.08 and 0.15 μmol·L-1, respectively, change the chirality to obtain compounds (R, S)-16a and (R, S)-17a, hERG The inhibitory activity decreased to 4.7 and 9.1 μmol·L-1, respectively. Therefore, changing the chirality of the drug is a modification idea that cannot be ignored in the modification of hERG.

2 The methyl strategy changes the conformation

The methylation strategy has a wide range of effects in the structural modification of medicines. The introduction of a carbon atom into the molecule can change the linearity of the molecule. Compound 18 is an H3 receptor antagonist with good activity in vitro (Ki = 7 nmol·L-1), but hERG inhibitory activity is strong (IC50 = 7 μmol·L-1), and a methylene group is introduced into the molecule , The molecule changes from broken line to linear, and compound 18a has a greatly reduced hERG inhibitory activity (IC50>30 μmol·L-1).

3 Increase molecular rigidity

Renin inhibitor 19 can be used for antihypertensive, but its hERG inhibitory activity is strong (IC50=5 μmol·L-1), and the incorporation of a spiro ring increases the molecular rigidity and successfully limits the rotation of the aromatic ring plane. Compound 19a hERG inhibits The activity decreased to less than 1/5 of 19 (IC50 = 24 μmol·L-1), and the renin inhibitory activity was significantly increased. This strategy cleverly achieved the backbone transition, reduced hERG inhibitory activity and obtained candidate compounds with novel structures.

Compound 20 is a leukotriene A4 hydrolase (LTA4H) inhibitor with strong hERG inhibitory activity (IC50=1.5 μmol·L-1). Using the above heterocyclic replacement strategy, the fat-soluble clogP is reduced, and compound 20a inhibits hERG. The activity decreased to 1/6 of 20 (IC50 = 8.9μmol·L-1). Based on this, the introduction of a rigid tropane structure can reduce the number of flexible conformations of the compound 20b molecule and further reduce the hERG inhibitory activity (IC50>10 μmol·L-1).

The ethyl quaternary ammonium salt of compound 21, chlorofieramine, has antiarrhythmic effects, but its hERG inhibitory activity is strong (IC50 = 4.3 nmol·L-1), and there are strong basic nitrogen and two longer flexible structures in the structure. Hydrophobic chain.

Louvel et al. studies have shown that basic nitrogen is important for anti-arrhythmic activity, so the effect of molecular skeleton rigidity and linking chain on hERG inhibitory activity was investigated. When the double bond is introduced, the compound 21a has no obvious effect on the conformational change, and it can still interact with the key amino acid residues Tyr652 and Phe656, and the hERG inhibitory activity is strong; when the alkynyl group is introduced, the compounds 21b-21c are obtained, and the molecule is rigid Increased, hERG inhibitory activity decreased to 1/13～1/110.

Summary and outlook

Reducing fat solubility, reducing alkalinity, introducing hydroxyl groups, introducing acidic fragments and conformational restrictions are five commonly used chemical structure modification strategies to reduce the inhibition of drugs on hERG potassium channels. Due to the close relationship between fat solubility and hERG inhibition, it has become the preferred structural optimization strategy for reducing hERG inhibitory activity, and it is also the most widely used. The second most common application is to reduce the basic pKa of the drug molecule. It is worth noting that the two methods of reducing the alkalinity and reducing the fat solubility are inherently related and can affect each other.

Reasonable introduction of hydrogen bond donors (hydroxyl groups) containing oxygen atoms can significantly reduce hERG inhibitory activity. The introduction of acidic fragments can significantly change the physical and chemical properties of drug molecules, resulting in a significant reduction in hERG inhibitory activity, but the limitation is that it may affect bioavailability and brain permeability.

Conformational restriction is a strategy that has been gradually developed and expanded in recent years. It involves new methods such as changing chirality, introducing methyl groups, backbone transitions, and increasing rigidity. Such modifications have little change in physical and chemical properties, and hinder drugs by changing the conformation or limiting the number of conformations. The interaction between molecules and hERG potassium channels provides new research ideas for the structural modification of lead compounds.

With more and more research reports on the structure-activity relationship (SAR) between drugs and hERG potassium channels and the development of computational chemistry, the accuracy of prediction models has improved, and CADD research strategies will be more accurate for compound design and optimization, and avoid hERG toxicity. Guidance.

In the future, the lead compound structure optimization strategy to reduce the hERG cardiotoxicity of the drug will be used in combination with pharmacophore model prediction, homology modeling, and molecular docking strategies to interact and assist each other, providing more rationality for reducing potential cardiotoxicity in the development of new drugs And rich reform ideas.