**1. Introduction**

Tiropramide is a drug that has been used from the past to the present for symptom relief and treatment of diseases of the digestive system such as acute spastic abdominal pain and irritable bowel syndrome. Tiropramide is a structurally equivalent tyrosine derivative and contains amide functional groups in the structure together with tertiary amines. It has been reported that tiropramide is widely metabolized into various metabolites (hydroxytiropramide, *N*-despropyltiropramide, *N*-desethyltiropramide, and *N*-desethyl-*N*-despropyltiropramide) after oral administration to rats and humans [1–3]. The mechanism of action of tiropramide has been reported to directly affect the smooth muscle cells. That is, tiropramide activates intracellular cyclic adenosine monophosphate (cAMP) synthase (adenylcyclase) to increase the amount of cAMP to regulate calcium ions necessary for muscle contraction, thereby controlling the relaxation and contraction of intestinal smooth muscle cells. This mechanism of action is very important for clinical use. Other antispasmodics include anticholinergics or antimuscarinics that relieve abdominal pain by acting on the intestinal nervous

system. However, the mechanism of action on the nervous system may cause systemic side effects such as dry mouth, dry eyes, and drowsiness, which may lower the patients' compliance with medication. As a result, tiropramide has been widely used in the clinic for the treatment of diseases related to the digestive system with the reduction of the systemic side effects.

However, reporting of pharmacokinetic (PK) information of tiropramide for humans is still very limited. Above all, tiropramide's population pharmacokinetics (PPK) have not been reported thus far. Tiropramide has been reported to have hypotension and peripheral vasodilation (as side effects) like other anticholinergic or antimuscarinic drugs only when a high dose is administered. Overall, incidences of tiropramide side effects are very low and a very safe profile in humans has been reported [4]. Although the incidence of adverse effects of tiropramide is reported to be low in humans, optimal dosing algorithm may maximize the therapeutic effect of the drug and reduce its adverse effects by using a PPK model. PPK modeling can enable effective dose setting and individualized pharmacotherapy by quantifying the diversity of the drug concentrations among individuals (in the population) with a variety of related physicochemical factors. In addition, identifying the physicochemical factors affecting PKs of tiropramide will be a significant scientific basis for the clinical application (such as usage and dose settings) and formulation of tiropramide in the future.

Tiropramide is usually administered orally to adults at 100 mg (1 tablet) 2-3 times a day in the clinic. In exceptional cases, it is reported that tiropramide may be additionally administered in cases of less symptomatic relief. However, it is difficult to judge whether these levels of capacity and usage are precise when considering the differences among individuals in the population. More scientific evidence and data on the dose setting and safety of tiropramide are needed. Therefore, we thought that a study of the PPK model of tiropramide was necessary. In addition, it begs the question on how the various physicochemical or genetic factors among individuals in the population affect the PKs of tiropramide. Moreover, even if individual physicochemical or genetic factors affect tiropramide's PKs, it is very doubtful as to how large the effect would be. Finding significant covariates of tiropramide is very difficult due to the lack of detailed PK information on the precise metabolic mechanisms, absorption, distribution, and excretion of tiropramide (especially in humans), and no PPK studies have been reported in the past. Therefore, in this study, we set up a variety of candidate covariates early in the study to establish factors that significantly influence the PK of tiropramide. The process was based on the physicochemical or physiological properties of tiropramide that have been reported to date.

Tiropramide has amine groups in its structure and will be basic at pH (about 7.4) in vivo on the basis of its pKa value (of 3.1) [4]. Therefore, the covariate effect was confirmed by genotyping the *OCT* gene, which is known to be related to the absorption, distribution, and excretion of various organic cations [5]. Tiropramide is a substance derived from the amino acid tyrosine [6]. Therefore, the genotyping of the *PEPT* gene, which is known to be involved in the absorption and distribution of the peptide drugs such as peptides and β-lactam antibiotics, was performed to confirm the covariate effect [7]. P-glycoprotein (P-gp) is a transporter with a broad range of substrate specificities of about 170 kDa and is known to be mainly involved in the efflux of neutral or cationic substances. In addition, *ABCB1* has been reported as a gene that encodes the P-gp. In this study, the covariate effect was confirmed by genotyping *ABCB1* [8]. In the past, the metabolism of tiropramide in the liver was examined [1], and the covariate effect was confirmed by genotyping metabolism-related *CYP2D6*. In particular, we focused on single nucleotide polymorphism (SNP) of the *CYP2D6* gene related to phase I metabolism (oxidation, reduction, hydrolysis, etc.) in the liver, and tried to confirm the correlation with PKs [9]. In addition, we collected information on various physicochemical factors (including general functional indicators of the kidney and liver) and sought to find the major covariates affecting PKs.

We report on PPK modeling of tiropramide in this study, together with factors affecting PK diversity in the population, which have not been reported yet. In the tiropramide final PPK model (of this study), we quantitatively reflected on the total protein, physicochemical, or genetic factors with differences between individuals, suggesting that scientific dose setting can be possible. In these

aspects, for clinical applications, the tiropramide's PPK model would be a great advantage. As a result, development of a tiropramide's PPK model for use in healthy Korean subjects was the purpose of this study. The developed tiropramide's PPK model is expected to be useful for determining a valuable dosing algorithm for tiropramide in healthy Korean subjects. In addition, the identification of factors affecting PK of tiropramide is expected to be of great help in related future studies.
