The study of intestinal absorption and biodistribution in vivo of proton pump inhibitors

Abstract

The major therapeutic strategy for acid-related gastrointestinal diseases in clinic is to reduce the excretion of gastric acid by oral administration of proton-pump inhibitors (PPIs). However, it is quite a challenge to study the oral absorption behaviors of PPIs considering their extreme instability under gastrointestinal environment. As a result, little information has been reported on PPI oral absorption so far, hindering the further development of PPI-contained oral preparations. Here, we first investigated the degradation rate of three representative PPIs, including ilaprazole, ilaprazole sodium and rabeprazole sodium. Then a modified in situ intestine absorption method in rat was established: through the temperature control by the heat exchangers, the perfusate was kept at physiological temperature only when passing through the intestine while it was maintained at 4 °C outside the intestine. Therefore PPIs could maintained sufficiently high stability under proper temperature control. Our data demonstrated that both ilaprazole and ilaprazole sodium exhibited significantly higher absorption efficiency than rabeprazole sodium did through the comparison of their apparent permeability coefficients and steady-state plasma concentrations after perfusion in the duodenum, jejunum, ileum and colon, mainly attributing to their more suitable oil-water partition coefficient. The duodenum could be the best site for the oral absorption of PPIs. Ilaprazole outperformed its sodium salt form with its stable absorption behavior in tested four intestinal segments. Furthermore, after intravenous or oral administration, ilaprazole exhibited a longer residence time and a higher accumulation in the stomach than in most of other tissues/organs. However, it was also found that the accumulation was heterogeneous and mainly located in mucosa cells of the stomach. Our further study indicated that there was no significant difference on the oral absorption efficiency of ilaprazole between female and male rats but ilaprazole underwent a faster metabolism in male rats after oral absorption. Our study provided a valuable guidance for the design of oral formulation and the optimization of PPI-contained formulations.

Introduction

Acid-related gastrointestinal diseases caused by excessive gastric acid secretion seriously lower the life quality of patients. Increased gastric acid can cause a direct damage to the esophagus, stomach and duodenal mucosa [1], [2]. In addition, high H⁺ concentration caused by excessive gastric acid can stimulate mast cells to secrete histamine, which in turn further induce the secretion of gastric acid through a paracrine action on parietal cells. Up-regulated histamine level can also stimulate local gastric tissue, causing its telangiectasia and increased permeability and eventually leading to congestion, edema, bleeding, erosion and even ulcers of gastric mucosa [3], [4], [5]. Currently, reducing the excretion of gastric acid by proton-pump inhibitors (PPIs) is regarded as the first-line therapeutic strategy for acid-related gastrointestinal diseases in clinic. PPIs become active after a combination with H⁺ under acidic condition and then bind to proton pump protein through the disulfide bond, resulting in a denaturation of the protein and subsequently a decreased H⁺ pumping [6], [7].

The PPIs pharmacologically fall into the categories of first-generation (as representative of omeprazole, lansoprazole and pantoprazole) and second-generation (including rabeprazole, esomeprazole, ilaprazole, leminoprazole etc). Compared to the first generation PPIs, the second one demonstrates many pharmacological advantages in clinic, such as shorter onset time, better acid inhibitory effect, no obvious nocturnal acid breakthrough (NAB), sustained acid suppression for 24 h, less individual differences and less interaction with other drugs [8], [9]. Different PPIs always behaves differently in vivo [7], [10], which is greatly related to their absorption characteristics in the gastrointestinal tract after oral administration. At present, the oral proton pump inhibitors such as omeprazole used in clinical practice are almost all enteric coating preparations [11], [12]. In academia, there are also reports on the absorption of omeprazole nanosolid oral preparations [13]. However, there are still more unknown information on the intestinal absorption behavior of PPIs so far. The main reason may be that the PPIs are extremely unstable in the gastrointestinal environment, according to our study: there is an obvious increase in drug degradation rate as pH decreases from 10.0 to 7.0, so as the temperature increases from 4 °C to 37 °C. For example, after incubation in Krebs-Ringer solution (pH = 7.8) for 2 h, 50% ilaprazole sodium was degraded at 37 °C, while only 3% was degraded at 4 °C and will be rapidly degraded even under physiological pH condition [14], increasing the difficulty for the development of oral absorption studies. A lack of information on PPI gastrointestinal absorption makes it difficult to explain the correlation between in vivo active behavior of various PPIs and their oral absorption. More importantly, insufficient data support induces an enormous challenge to the further design and optimization of new PPI oral formulations.

In situ intestine absorption model in rat is the most common choice to study oral absorption of drug molecules in the intestine, which stands out for it’s well simulation of the real absorption state in vivo and accurate reflection of the absorption characteristics of the molecules [15]. In this study, taking the extreme chemical instability of PPIs into consideration, we developed a modified in situ intestine absorption method in rat to investigate their absorption behavior, where the PPIs can exhibit sufficiently high chemical stability during the perfusion in the rat intestinal lumen. As representatives of the second-generation PPIs, ilaprazole and its sodium salt form (ilaprazole sodium) were selected to study the intestinal absorption behavior of PPIs with low and high solubility, respectively. To further confirm the absorption characteristics of ilaprazole, ¹⁴C labeled ilaprazole was synthesized and its absorption test was carried out using the same method. Furthermore, the biodistribution of ilaprazole was investigated by radioactivity counting and autoradiography after intravenous injection. The sodium salt form of rabeprazole (rabeprazole sodium), another second-generation PPI, serves as a control to study the absorption behavior of PPI with different molecular structure compared to ilaprazole sodium. Our study reveals for the first time the intestinal absorption characteristics of three 2nd-generation PPIs which are ilaprazole, ilaprazole sodium and rabeprazole sodium. More importantly, the method we developed makes it possible to study the intestinal absorption behavior of all current PPIs because they have similar chemical instability under physiological conditions, which will provide important guidance for formulation optimization and formulation development of PPIs.