Genetic polymorphisms of CYP2A6 affect the in-vivo pharmacokinetics of pilocarpine

The pharmacogenetics of natural products: A pharmacokinetic and pharmacodynamic perspective

Natural products have represented attractive alternatives for disease prevention and treatment over the course of human history and have contributed to the development of modern drugs. These natural products possess beneficial efficacies as well as adverse efffects, which vary largely among individuals because of genetic variations in their pharmacokinetics and pharmacodynamics. As with other synthetic chemical drugs, the dosing of natural products can be optimized to improve efficacy and reduce toxicity according to the pharmacogenetic properties. With the emergence and development of pharmacogenomics, it is possible to discover and identify the targets/mechanisms of pharmacological effects and therapeutic responses of natural products effectively and efficiently on the whole genome level. This review covers the effects of genetic variations in drug metabolizing enzymes, drug transporters, and direct and indirect interactions with the pharmacological targets/pathways on the individual response to natural products, and provides suggestions on dosing regimen adjustments of natural products based on their pharmacokinetic and pharmacogenetic paratmeters. Finally, we provide our viewpoints on the importance and necessity of pharmacogenetic and pharmacogenomic research of natural products in natural medicine's rational development and clinical application of precision medicine.

Exploring the structure characteristics and major channels of cytochrome P450 2A6, 2A13, and 2E1 with pilocarpine

The majority of cytochromes P450 play a critical role in metabolism of endogenous and exogenous substrates, some of its products are carcinogens. Therefore, inhibition of P450 enzymes activity can promote the detoxification and elimination of chemical carcinogens. In this study, molecular dynamics (MD) simulations and adaptive steered molecular dynamics (ASMD) simulations were performed to explore the structure features and channel dynamics of three P450 isoforms 2A6, 2A13, and 2E1 bound with the common inhibitor pilocarpine. The binding free energy results combined with the PMF calculations give a reasonable ranking of binding affinity, which are consistent with the experimental data. Our results uncover how a sequence divergence of different CYP2 enzymes causes individual variations in major channel selections. On the basis of channel bottleneck and energy decomposition analysis, we propose a gating mechanism of their respective major channels in three enzymes, which may be attributed to a reversal of Phe209 in CYP2A6/2A13, as well as the rotation of Phe116 and Phe298 in CYP2E1. The hydrophobic residues not only make strong hydrophobic interactions with inhibitor, but also act as gatekeeper to regulate the opening of channel. The present study provides important insights into the structure-function relationships of three cytochrome P450s and the molecular basis for development of potent inhibitors.

Variation in CYP2A6 Activity and Personalized Medicine

The cytochrome P450 2A6 (CYP2A6) enzyme metabolizes several clinically relevant substrates, including nicotine-the primary psychoactive component in cigarette smoke. The gene that encodes the CYP2A6 enzyme is highly polymorphic, resulting in extensive interindividual variation in CYP2A6 enzyme activity and the rate of metabolism of nicotine and other CYP2A6 substrates including cotinine, tegafur, letrozole, efavirenz, valproic acid, pilocarpine, artemisinin, artesunate, SM-12502, caffeine, and tyrosol. CYP2A6 expression and activity are also impacted by non-genetic factors, including induction or inhibition by pharmacological, endogenous, and dietary substances, as well as age-related changes, or interactions with other hepatic enzymes, co-enzymes, and co-factors. As variation in CYP2A6 activity is associated with smoking behavior, smoking cessation, tobacco-related lung cancer risk, and with altered metabolism and resulting clinical responses for several therapeutics, CYP2A6 expression and enzyme activity is an important clinical consideration. This review will discuss sources of variation in CYP2A6 enzyme activity, with a focus on the impact of CYP2A6 genetic variation on metabolism of the CYP2A6 substrates.

Pharmacogenomics in Asia: a systematic review on current trends and novel discoveries

While early pharmacogenomic studies have primarily been carried out in Western populations, there has been a notable increase in the number of Asian studies over the past decade. We systematically reviewed all pharmacogenomic studies conducted in Asia published before 2016 to highlight trends and identify research gaps in Asia. We observed that pharmacogenomic research in Asia was dominated by larger developed countries, notably Japan and Korea, and mainly driven by local researchers. Studies were focused on drugs acting on the CNS, chemotherapeutics and anticoagulants. Significantly, several novel pharmacogenomic associations have emerged from Asian studies. These developments are highly encouraging for the strength of regional scientific and clinical community and propound the importance of discovery studies in different populations.

The Sustained Effects on Tear Volume of Pilocarpine Hydrochloride in Gelatin by Hydrogel Administered by An Implant-mediated Drug Delivery System

BACKGROUND

Pilocarpine hydrochloride is commonly prescribed to patients with dry mouth and eye using a frequent dosing schedule. The aim of this study was to evaluate the sustained effects of this highly soluble drug carried by a gelatin hydrogel, which was administered by an implant mediated drug delivery system (IMDDS).

METHODS

The IMDDS was installed in a total of 24 rabbits. After complete healing, pilocarpine hydrochloride was administered as 30 mg as raw powder (Group 1; n = 8), 30 mg in gelatin hydrogel (Group 2; n = 8), and 60 mg in gelatin hydrogel (Group 3; n = 8). The effects were evaluated by tear volume measured using the Schirmer tear test for 2 weeks after administration.

RESULTS

All 3 groups showed an increase in tear volume from the initial measurement at 1 hour. Group 1 exhibited this increase for 24 hours, while Groups 2 and 3 sustained this increase for 5 days and 7.5 days, respectively.

CONCLUSION

When provided in gelatin hydrogel, highly water-soluble pilocarpine hydrochloride administered through IMDDS resulted in sustained effects with increased tear volume in normal rabbits.

Effect of CYP2A6 genetic polymorphism on the metabolic conversion of tegafur to 5-fluorouracil and its enantioselectivity

Tegafur (FT), a prodrug of 5-fluorouracil, is a chiral molecule, a racemate of R- and S-isomers, and CYP2A6 plays an important role in the enantioselective metabolism of FT in human liver microsomes (R-FT >> S-FT). This study examined the enantioselective metabolism of FT by microsomes prepared from Sf9 cells expressing wild-type CYP2A6 and its variants (CYP2A6*7, *8, *10, and *11) that are highly prevalent in the Asian population. We also investigated the metabolism of coumarin and nicotine, both CYP2A6 probe drugs, in these variants. Enzyme kinetic analyses showed that CYP2A6.7 (I471T) and CYP2A6.10 (I471T and R485L) had markedly lower Vmax values for both enantiomers than wild-type enzyme (CYP2A6.1) and other variant enzymes, whereas Km values were higher in most of the variant enzymes for both enantiomers than CYP2A6.1. The ratios of Vmax and Km values for R-FT to corresponding values for S-FT (R/S ratio) were similar among enzymes, indicating little difference in enantioselectivity among the wild-type and variant enzymes. Similarly, both CYP2A6.7 and CYP2A6.10 had markedly lower Vmax values for coumarin 7-hydroxylase and nicotine C-oxidase activities than CYP2A6.1 and other variant enzymes, whereas Km values were higher in most of the variant enzymes for both activities than CYP2A6.1. In conclusion, the amino acid substitutions in CYP2A6 variants generally resulted in lower affinity for substrates, while Vmax values were selectively reduced in CYP2A6.7 and CYP2A6.10. Consistent R/S ratios among CYP2A6.1 and variant enzymes indicated that the amino acid substitutions had little effect on enantioselectivity in the metabolism of FT.

Clinical safety, tolerability and efficacy of combination tolterodine/pilocarpine in patients with overactive bladder

AIMS

The purpose of this study was to assess the safety, tolerability and impact on overactive bladder (OAB) symptoms of a novel combination of tolterodine immediate-release (IR) 2 mg and delayed-release pilocarpine 9 mg in patients with OAB.

METHODS

Eligible patients with OAB were randomised to each of three treatments [tolterodine/pilocarpine (2/9 mg), tolterodine IR 2 mg or placebo] twice daily for 4 weeks in a double-blind, crossover fashion. At the end of the 12-week, double-blind treatment period, patients could enter an open-label extension during which they were re-randomised to either tolterodine/pilocarpine (3/13.5 mg) twice daily or tolterodine extended-release 4 mg once daily for 12 weeks.

RESULTS

A total of 138 patients were randomised to double-blind medication. Both tolterodine/pilocarpine (2/9) and tolterodine IR 2 mg significantly reduced incontinence episodes and daily micturitions (p < 0.001 vs. placebo), with similar reductions in symptoms observed between active treatment groups. Tolterodine/pilocarpine (2/9) was associated with consistently lower Visual Analogue Scale (VAS) scores for all dry mouth parameters compared with tolterodine alone. Salivary flow over a 3 h period remained fairly constant after tolterodine/pilocarpine (2/9) administration, similar to placebo, but decreased markedly after administration of tolterodine alone. In the extension study, patients receiving tolterodine/pilocarpine (3/13.5) reported comparable dry mouth VAS scores to tolterodine extended-release alone without additional side effects or loss of efficacy. The combination was well tolerated, and the adverse effects observed were consistent with the known safety profiles of tolterodine and pilocarpine.

CONCLUSIONS

A combination of tolterodine/pilocarpine (2/9) effectively reduced the incidence of dry mouth compared with tolterodine IR alone while maintaining treatment efficacy in OAB.

Human paraoxonase 1 is the enzyme responsible for pilocarpine hydrolysis

Pilocarpine has been widely used in ophthalmic preparations for the treatment of glaucoma and in oral preparations for the treatment of radiation-induced xerostomia and Sjögren syndrome. The major metabolic pathways of pilocarpine in human are hydrolysis and hydroxylation. It was found that CYP2A6 is responsible for the 3-hydroxylation, but the enzymes responsible for the hydrolysis have not been characterized. In this study, we attempted to identify esterases responsible for pilocarpine hydrolysis. Pilocarpine hydrolase activities in human liver microsomes and plasma were stimulated by the addition of CaCl(2), suggesting that the calcium-dependent esterase, paraoxonase (PON), was responsible for pilocarpine hydrolysis. To confirm this hypothesis, the pilocarpine hydrolase activity was measured using the recombinant human PONs (PON1, PON2, and PON3) established in this study, and the result was that only PON1 showed pilocarpine hydrolase activity. The effect of PON1 polymorphism (Q192R) on pilocarpine hydrolase activity was analyzed using recombinant human PON1 192Q and 192R and human plasma from 50 volunteers. The results showed that recombinant PON1 192R revealed significantly higher catalytic efficiency than PON1 192Q. In human plasma, the activity of the R/R genotype (117.0 ± 25.2 pmol · min(-1) · μl(-1), n = 23) was significantly higher than those of the Q/R and Q/Q genotypes (97.3 ± 21.0 pmol · min(-1) · μl(-1), n = 20 and 90.4 ± 26.2 pmol · min(-1) · μl(-1), n = 7, respectively). It is suggested that this polymorphism affects pilocarpine hydrolase activity. In this study, we found that human PON1 is the major enzyme for the catalytic efficiency of pilocarpine hydrolysis.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.