Efficacy of Naldemedine on Intestinal Hypomotility and Adhesions in Rodent Models of Postoperative Ileus

Abstract

Postoperative ileus (POI) often decreases patients’ QOL because of prolonged hospitalization and readmission. Alvimopan, a peripheral μ-opioid receptor antagonist, is currently the only therapeutic drug for POI. The aim of this study was to examine the efficacy of naldemedine (a peripheral μ-opioid receptor antagonist with a non-competitive pharmacological profile different from that of alvimopan) on postoperative intestinal hypomotility and adhesion in rodent models, and compare it with the effects of alvimopan. Oral administration of naldemedine (0.3 mg/kg) and alvimopan (3 mg/kg) significantly inhibited the decrease in intestinal motility induced by mechanical irritation in mice (p < 0.01, for both). Naldemedine (1 mg/kg) significantly shortened the adhesion length in chemical-induced postoperative adhesion model rats (p < 0.05). Alvimopan (3 mg/kg) also significantly reduced the adhesion ratio (p < 0.01). These findings suggest that naldemedine is effective for postoperative intestinal hypomotility and adhesions in rodents (i.e., as for alvimopan). Thus, naldemedine may be a useful option for the treatment of POI.

INTRODUCTION

Intestinal peristalsis plays a critical role in the absorption of necessary nutrients and water, and in the excretion of waste substances including stool.¹⁾ However, intestinal peristalsis is often affected by medication and surgery, leading to intestinal hypomotility.^2–4) Postoperative ileus (POI) is a particularly difficult disease to manage, and is characterized by temporary hypomotility of the gastrointestinal tract after abdominal surgery.³⁾ POI occurs in >60% of patients with laparotomy,⁵⁾ while other causes include hernia, malignancy, and inflammation. Endogenous/exogenous opioids can also inhibit gastrointestinal motility, leading to ileus in patients with surgery.^6,7) The treatment of POI mainly involves conventional therapies such as enteral nutrition, infusion drip, and electrolyte manipulation, as well as physical activity and training.¹⁾ However, the efficacy of these treatments in POI patients is poor, and new treatment options are required.¹⁾

The majority of patients with laparotomy experience intestinal adhesions^5,8) and lead to the development of POI.⁷⁾ Interestingly, a large-scale retrospective study shows that patients receiving laparoscopic surgery have fewer readmissions that are directly or possibly related to adhesions compared with patients receiving laparotomy.⁹⁾ Improving the surgical procedures and development of novel clinical treatment options are required to effectively reduce the social burden of readmissions after surgery for POI.

Alvimopan is the only U.S. Food and Drug Administration (FDA)-approval drug for POI as a peripheral μ-opioid receptor antagonist (PAMORA).¹⁰⁾ Several clinical studies have reported that alvimopan shortens the time until recovery of defecation and decreases the duration of POI and hospitalization.^2,10) The effect of alvimopan on POI has also been investigated in non-clinical studies.¹¹⁾ Naldemedine is a PAMORA with a non-competitive pharmacological profile in the treatment of opioid-induced constipation.¹²⁾ However, the effect of naldemedine on POI has not been investigated. The aim of this study was to assess the efficacy of naldemedine on postoperative intestinal motility and adhesion in rodent models by comparing it with the effects of alvimopan.

MATERIALS AND METHODS

Drugs

Naldemedine (Shionogi & Co., Ltd., Osaka, Japan) and alvimopan (Sigma-Aldrich, St. Louis, MO, U.S.A.) were dissolved in 0.5% methyl cellulose solution (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan). All drugs were orally administered at a volume of 10 or 2 mL/kg for mice or rats, respectively. For evaluation of adhesion, all drugs were orally administered twice daily for 2 d after first administration at 1 h (naldemedine) or 2 h (alvimopan) after surgery. At 3 d after surgery, naldemedine and alvimopan were administered at 1 and 2 h before evaluation of intestinal adhesions, respectively.

Animals

C57BL/6J male mice (9–11 weeks old at the start of experiments) were obtained from Japan SLC, Inc. (Shizuoka, Japan) and used to develop a POI-like model. Sprague–Dawley male rats (6 weeks old at the start of experiments) were obtained from the Jackson Laboratory Japan, Inc. (Kanagawa, Japan) and used to develop a postoperative adhesion (POA)-like model. Mice and rats were housed in a room maintained at 23 ± 1 °C under a 12-h light/dark cycle with ad libitum access to food (CLEA Rodent Diet CE-2, CLEA Japan, Inc., Tokyo, Japan) and tap water. All procedures for the animal experiments were approved by the Animal Care and Use Committee of Shionogi Research Laboratories (Osaka, Japan) and were performed according to the Association for Assessment and Accreditation of Laboratory Animal Care International (AAALAC) guidelines.

Experimental Models

Mechanical-Induced Obstruction Model

To develop POI-like mechanical-induced obstruction model mice (MIO mice), C57BL/6J male mice were used and surgery was performed as previously reported, with modifications.¹³⁾ In brief, all mice were anesthetized with 2% isoflurane for 20–25 min during the operation. The abdominal skin of each animal was shaved with an electric hair clipper the day before surgery, and cleaned with povidone-iodine just before the operation. A 3-cm-long incision was made approximately 5 mm adjacent to the abdominal midline. The distal ileum (10 cm from the ileocecal valve) was exposed and rubbed very gently (10 times for 5 min) by mechanical blunt stimulation using a sterile cotton swab moistened with physiological saline (Otsuka Holdings Co., Ltd., Tokushima, Japan). The abdomen was then closed with 4-0 monofilament sutures (Natsume Seisakusho Co., Ltd., Tokyo, Japan). Mice in the sham group received the abdominal incision but not gut manipulation. After waking from anesthesia, mice were fasted for 20 h under free access to water. The number of animals was decided based on our preliminary study. Sixteen mice were estimated for mechanical-induced obstruction model (intergroup difference = 0.9, standard deviation = 0.9)

Chemical-Induced Adhesion Model

The POA-like chemical-induced adhesion model rats (CIA rats) were developed in Sprague–Dawley male rats as previously reported.¹⁴⁾ In brief, under anesthesia with 2% isoflurane, the abdominal skin of each animal was shaved with an electric hair clipper and cleaned with povidone-iodine. A 3-cm-long incision was performed approximately 5 mm adjacent to the abdominal midline, and the small intestine was then exposed. Small scratches were made in the small intestine using a toothbrush, and talc (60 mg; Nacalai Tesque, Inc., Kyoto, Japan) was uniformly dispersed over the region. The small intestine was then replaced and the wound closed. After waking from anesthesia, the rats were fasted for at least 16 h under free access to water.

The rats were then administered naldemedine (1, 3, 10 mg/kg) or alvimopan (3 and 10 mg/kg). At 3 d after drug treatment, the abdomen was reopened and the adhesion condition was evaluated, as previously reported.^14,15) The evaluation method is shown in “Evaluation of adhesion.” In each experimental round, two or three rats from each group were evaluated, with a final total of 20 rats per group. Twenty rats were estimated for chemical-induced adhesion model (intergroup difference = 10, standard deviation = 9) at the 0.05 alpha level (power = 0.8).

Evaluation of Intestinal Transit

The day after surgery, naldemedine (0.1, 0.3, or 1 mg/kg), alvimopan (3 mg/kg), or vehicle (0.5% methyl cellulose solution) were administered orally at a volume of 10 mL/kg. Fluorescein isothiocyanate (FITC)-dextran (Sigma-Aldrich) dissolved at 5 mg/mL in distilled water (Otsuka Pharmaceutical Factory Inc., Tokushima, Japan) was administered orally at a volume of 0.1 mL in mice at 30 min after drug administration. Mice were then euthanized by cervical dislocation under anesthesia with 2% isoflurane at 90 min after FITC-dextran administration. The entire gastrointestinal tract was then separated into segments consisting of the stomach, 10 equally sized small intestinal segments (S2–S11), cecum, and three equally sized large intestinal segments. Each segment was mixed with 2 mL of physiological saline to collect the FITC-dextran, and samples were then centrifuged at 526× g for 10 min. For each segment, the FITC-dextran of the cleared supernatant was measured in a plate reader (ARVO MX-3; PerkinElmer, Inc., Waltham, MA, U.S.A.) at excitation and emission wavelengths 485 and 535 nm, respectively (Fig. 1A). The transit was visualized by plotting the FITC-dextran line graph and summarized by calculating the geometric center (GC) of the distribution as Σ (percentage of total fluorescence per segment × number of segments)/100.

Fig. 1. Effect of Naldemedine on Intestinal Motility in Mechanical-Induced Obstruction Model Mice (MIO Mice)

Naldemedine (0.1, 0.3, or 1 mg/kg), alvimopan (3 mg/kg), or vehicle were orally administered at a volume of 10 mL/kg on the day after surgery. After 30 min post-administration, fluorescein isothiocyanate (FITC)-dextran was administered orally. The entire gastrointestinal tract was isolated at 90 min after FITC-dextran administration, and was then separated into stomach segments. (A) The geometric center (GC) was measured using FITC-dextran and calculated as GC = Σ (% of total fluorescence per segment × number of segments)/100. Data are presented as (B) mean ± standard error for % distribution and (C) the GC of FITC-dextran (n = 16). Data were analyzed by Student’s t-test (^††p < 0.01) and Dunnett’s test (** p < 0.01) compared with vehicle.

Evaluation of Adhesion

At 3 d after the operation, rats were euthanized by cervical dislocation under anesthesia with 2% isoflurane. The abdomen was opened and various adhesion parameters were calculated, as follows: total adhesion length = sum of adhesion lengths per small intestine; adhesion ratio (%) = total adhesion length/total small intestine length × 100; and adhesion length per adhesion (adhesion size) = total adhesion lengths/number of adhesions per small intestine ×100.

Measurement of Naldemedine Plasma Concentrations in Mice

The plasma concentrations of naldemedine measured in MIO mice.¹⁶⁾ Whole blood (approximately 0.4 mL) was collected from the abdominal vena cava or heart under anesthesia with 2% isoflurane using a syringe containing 10 µL heparin sodium (250 unit/mL). Collected blood samples were immediately chilled on ice and centrifuged at 526 × g for 15 min. The plasma concentration of naldemedine was determined by Liquid chromatography-tandem mass spectrometry, as previously described.¹⁶⁾

Statistical Analysis

All data are presented as the mean ± standard error for each group. Student’s t-test was used for non-paired samples. For multiple comparisons, Dunnett’s test, or Steel test were used. Statistical significance of post-hoc comparisons was defined as p < 0.05. All statistical analyses were performed using statistical software (GraphPad Prism v6.0; San Diego, CA, U.S.A.).

RESULTS

Effects of Naldemedine on Decreased Intestinal Motility in MIO Mice

To evaluate the intestinal transit in mice, the transit of FITC-dextran was measured in the intestine (Fig. 1B) and calculated as a GC (Fig. 1C). In sham-treated mice, the FITC-dextran was transported down the intestine, which peaked at segment S9 of the small intestine. In vehicle-treated MIO mice, the FITC-dextran transit peaked at segment S3, while the GC was 3.41 ± 0.10 compared with 7.35 ± 0.36 in sham-treated mice. Naldemedine at doses of 0.3 and 1 mg/kg significantly reversed the decrease in FITC-dextran transit in mice, with GCs of 3.98 ± 0.14 (p < 0.01) and 4.26 ± 0.13 (p < 0.01), respectively. These effects of naldemedine were comparable with that of alvimopan at 3 mg/kg (GC = 4.05 ± 0.16; Figs. 1B, C).

Pharmacokinetics of Naldemedine in MIO Mice

The plasma concentration of naldemedine was measured following oral naldemedine administration in mice (Table 1). In sham-treated mice receiving 0.3 mg/kg naldemedine, the plasma naldemedine concentrations were peaked at 0.5 h post-administration (77.4 ± 8.2 ng/mL) and gradually decreased thereafter. In MIO mice, the plasma concentration of naldemedine was peaked at 0.5 h post-administration at 0.1 mg/kg or at 1 h post-administration at 0.3 and 1 mg/kg with a dose-dependent increase (30.2 ± 13.7 ng/mL at 0.1 mg/kg, 71.7 ± 11.1 ng/mL at 0.3 mg/kg, and 316.3 ± 123.9 ng/mL at 1 mg/kg).

Table 1. Pharmacokinetics of Naldemedine in Sham-Treated (Sham) and Mechanical-Induced Obstruction Model Mice (MIO Mice)

	Naldemedine dose	0.5 (h)	1 (h)	2 (h)	4 (h)
Naldemedine (ng/mL)	MIO-0.1 mg/kg	30.2 ± 13.7	23.8 ± 4.0	15.0 ± 4.5	3.3 ± 0.5
	MIO-0.3 mg/kg	66.3 ± 27.7	71.7 ± 11.1	45.3 ± 19.0	24.7 ± 8.4
	MIO-1 mg/kg	215.3 ± 43.5	316.3 ± 123.9	147.3 ± 39.7	67.5 ± 43.8
	Sham-0.3 mg/kg	77.4 ± 8.2	56.2 ± 18.1	44.8 ± 6.3	33.4 ± 7.3

Data are presented as mean ± standard error (n = 3). There were no differences in pre-modeling or pre-drug body weights between the groups. The plasma concentration of naldemedine was determined by liquid chromatography-tandem mass spectrometry.

Effect of Naldemedine on Adhesions in CIA Rats

Adhesions were observed from 1 d post-operation in CIA rats, and the adhesion ratio was maintained until at least 5 d post-operation (Fig. 2A; 36.4 ± 2.1 at day 1, 39.9 ± 3.0 at day 3, and 37.5 ± 2.6 at day 5). To evaluate the efficacy of test drugs, they were orally administered twice a day for 3 d after surgery. Oral administration of naldemedine reduced adhesions at 3 d post-operation (Fig. 2B), with a significant reduction in the adhesion length in rats administered 1 mg/kg naldemedine compared with vehicle (Table 2; vehicle = 4.5 ± 0.3 cm, naldemedine = 3.3 ± 0.3 cm; p = 0.02). Naldemedine tended to reduce the adhesion ratio at 1 mg/kg, but not affect the total number of adhesions. By contrast, 3 mg/kg alvimopan significantly reduced the adhesion ratio (Table 2; alvimopan = 36.4 ± 2.3, vehicle = 46.5 ± 1.8; p < 0.01). However, there was no significant effect of alvimopan on adhesion length (Table 2).

Fig. 2. Effect of Naldemedine on Adhesions in Chemical-Induced Adhesion Model Rats (CIA Rats)

(A) Time course of development of adhesions in CIA rats. Data are presented as mean ± standard error (n = 15) and were analyzed by Dunnett test (*** p < 0.001) compared with sham-treated rats. (B) Representative photo showing the effect of 10 mg/kg naldemedine on adhesion in CIA rats. Circle shows mass of adhesions.

Table 2. Effects of Naldemedine and Alvimopan on Adhesion in Chemical-Induced Adhesion Model Rats (CIA Rats)

Treatment	Total adhesion lengths (cm)	Intestine length (cm)	Adhesion ratio (%)	Number of adhesions (count/rat)	Adhesion length per adhesion (adhesion size) (cm)
Vehicle	32.8 ± 1.6	76.9 ± 0.6	42.8 ± 2.1	8.3 ± 0.6	4.5 ± 0.3
Naldemedine 1 mg/kg	29.3 ± 1.8	78.4 ± 0.5	37.3 ± 2.3 (0.26)	9.2 ± 0.7 (0.59)	3.3 ± 0.3 (0.02)*
Naldemedine 3 mg/kg	31.7 ± 1.7	78.8 ± 0.7	40.2 ± 2.1 (0.64)	8.9 ± 0.6 (0.81)	3.8 ± 0.3 (0.26)
Naldemedine 10 mg/kg	27.9 ± 1.1	78.1 ± 0.7	35.8 ± 1.5 (0.01)§	9.9 ± 0.7 (0.19)	3.1 ± 0.2 (<0.01)**
Vehicle	36.4 ± 1.3	78.4 ± 0.9	46.5 ± 1.8	9.5 ± 0.6	4.4 ± 0.5
Alvimopan 3 mg/kg	27.5 ± 1.8	75.1 ± 0.6	36.4 ± 2.3 (<0.01)§§	8.1 ± 0.6 (0.17)	3.9 ± 0.5 (0.63)
Alvimopan 10 mg/kg	31.0 ± 1.6	78.3 ± 0.7	39.6 ± 1.9 (0.06)	10.1 ± 0.5 (0.69)	3.2 ± 0.3 (0.12)

Data are presented as mean ± standard error (n = 20). Test drugs were orally administered twice a day for 3 d after surgery. There were no differences in pre-modeling or pre-drug body weights between the groups. Total adhesion length = sum of adhesion lengths per small intestine; Adhesion ratio (%) = total adhesion length/total small intestine length ×100; Adhesion length per adhesion (adhesion size) = total adhesion lengths/number of adhesions per small intestine ×100. Statistical analysis was conducted using the Steel test for adhesion ratio and Dunnett’s test for number of adhesions and adhesion length per adhesion. The numbers in parentheses refer to p-values. * p < 0.05, ** p < 0.01 vs. vehicle treated groups (Dunnett’s test). ^§p < 0.05, ^§§p < 0.01 vs. vehicle treated group (Steel test).

DISCUSSION

In the present study, we found that naldemedine treatment improved intestinal motility reduction in MIO mice and prevented the development of adhesions in CIA rats. Alvimopan also improved the intestinal hypomotility in MIO mice, as previously reported in rats,¹¹⁾ and reduced adhesions in CIA rats. Overall, these findings suggest that PAMORAs could improve intestinal motility with a reduction of adhesion in rodent models.

We found that the efficacy and the effective doses of naldemedine in this study were partial and high (i.e., 0.3 mg/kg in MIO mice and 1 mg/kg in CIA rats) compared to those in opioid-induced small intestinal transit inhibition, which was significantly reduced by 0.03 mg/kg in rats.¹⁷⁾ In the present study, while the peak time of plasma concentration of naldemedine was delayed, there were no differences in the maximum concentration of naldemedine between MIO and sham mice, suggesting that intestinal impairment does not affect naldemedine absorption. Since POI is caused by several factors including endogenous opioids, inflammatory mediators, hormones, and fluid balance,^18,19) other mediators such as inflammatory cytokines would be involved in the pathology of MIO mice and CIA rats.⁶⁾ Alvimopan and naldemedine are thought to produce an antagonistic effect on the endogenous opioids induced by abdominal stimulation, thereby improving intestinal motility reduction and adhesions. The dosage of alvimopan in the present study was slightly higher, but almost equivalent to those used clinically in POI,^20,21) which suggests that higher drug concentrations may be required for the suppression of POI compared with opioid-induced constipation. During laparotomy, the release of cytokines (via the invasive action) could stimulate the secondary production of endogenous opioids.²²⁾ Endogenous morphine, which is immediately elevated in plasma after surgery for several hours, can directly activate the μ-opioid receptor in the enteric nervous system and indirectly via stress hormones.^23,24) Under these circumstances, continuous inhibition of μ-opioid receptors with high concentration of PAMORA might be required for suppression of POI and adhesions. Further studies are required to examine the involvement of endogenous opioids in MIO mice and CIA rats. In addition, as a limitation of this study, we used inbred mouse strain only in MIO mice and we cannot account for variability in results due to outbred. Further studies using outbred strains will be needed.

In CIA rats, naldemedine and alvimopan showed efficacy at the lowest dose studied, but clear dose-dependency were not observed. Also, naldemedine and alvimopan showed slightly different pattern of inhibition for adhesion, naldemedine reduced adhesion size, whereas alvimopan reduced adhesion ratio. Note that naldemedine is reported to show highly selective antagonistic activity against μ-, δ-, and κ-opioid receptors,¹²⁾ while alvimopan has selective antagonistic activity against μ-opioid receptor.²⁵⁾ Since δ-, and κ-opioid receptors are involved in the intestinal motility suppression effect of opioids,²⁶⁾ this may contribute to the dose-dependency and differences of efficacy in naldemedine and alvimopan. In addition, the effects of alvimopan may also be attributed to its active metabolite, ADL 08-0011, which is high-affinity to μ-opioid receptor and has an up to 10-fold greater concentration than alvimopan with long duration.^27,28) Further studies will be required to ensure the involvement of each opioid receptors for the development of adhesion by using genetically modified animals.

Alvimopan promotes gastrointestinal recovery of inpatients treated with laparotomy, and reduces the patient burden including hospital stay and prognosis, even with short-term use.^3,4,29,30) Naldemedine has wide safety range and high convenience without the requirement for drug titration,¹²⁾ and our findings suggest that naldemedine may be an effective treatment option for POI. MIO mice and CIA rats used in this study show the properties of POI such as intestinal transport reduction and adhesion, but they are not fully validated by clinically approved POI drugs other than alvimopan. Further pharmacological studies would be needed to validate these rodent models.

In summary, naldemedine improved intestinal hypomotility and adhesions in POI-like and POA-like rodent models, respectively. These findings suggest that naldemedine may be a useful treatment option for POI.

Conflict of Interest

YA, KK, TY, TK, YM, and MF are employees of Shionogi & Co., Ltd. HC, AM, HT, and SK are employees of Shionogi TechnoAdvance Research Co., Ltd. TS has no conflict of interest to disclose.

REFERENCES

• 1) Mattei P, Rombeau JL. Review of the pathophysiology and management of postoperative ileus. World J. Surg., 30, 1382–1391 (2006).
• 2) Bream-Rouwenhorst HR, Cantrell MA. Alvimopan for postoperative ileus. Am. J. Health Syst. Pharm., 66, 1267–1277 (2009).
• 3) Holte K, Kehlet H. Postoperative ileus: a preventable event. Br. J. Surg., 87, 1480–1493 (2000).
• 4) Adam MA, Lee LM, Kim J, Shenoi M, Mallipeddi M, Aziz H, Stinnett S, Sun Z, Mantyh CR, Thacker JKM. Alvimopan provides additional improvement in outcomes and cost savings in enhanced recovery colorectal surgery. Ann. Surg., 264, 141–146 (2016).
• 5) Weibel MA, Majno G. Peritoneal adhesions and their relation to abdominal surgery: a postmortem study. Am. J. Surg., 126, 345–353 (1973).
• 6) Becker G, Blum H. Novel opioid antagonists for opioid-induced bowel dysfunction and postoperative ileus. Lancet, 373, 1198–1206 (2009).
• 7) Bragg D, El-Sharkawy AM, Psaltis E, Maxwell-Armstrong CA, Lobo DN. Postoperative ileus: recent developments in pathophysiology and management. Clin. Nutr., 34, 367–376 (2015).
• 8) Menzies D, Ellis H. Intestinal obstruction from adhesions—how big is the problem? Ann. R. Coll. Surg. Engl., 72, 60–63 (1990).
• 9) Krielen P, Stommel MWJ, Pargmae P, Bouvy ND, Bakkum EA, Ellis H, Parker MC, Griffiths EA, van Goor H, Ten Broek RPG. Adhesion-related readmissions after open and laparoscopic surgery: a retrospective cohort study (SCAR update). Lancet, 395, 33–41 (2020).
• 10) Hyde LZ, Kiely JM, Al-Mazrou A, Zhang H, Lee-Kong S, Kiran RP. Alvimopan significantly reduces length of stay and costs following colorectal resection and ostomy reversal even within an enhanced recovery protocol. Dis. Colon Rectum, 62, 755–761 (2019).
• 11) Fukuda H, Suenaga K, Tsuchida D, Mantyh CR, Pappas TN, Hicks GA, DeHaven-Hudkins DL, Takahashi T. The selective mu opioid receptor antagonist, alvimopan, improves delayed GI transit of postoperative ileus in rats. Brain Res., 1102, 63–70 (2006).
• 12) Kanemasa T, Koike K, Takase K, Arai T, Nakamura A, Morioka Y, Hasegawa M. Pharmacological profile of naldemedine, a peripherally acting μ-opioid receptor antagonist: comparison with naloxone and naloxegol. J. Pharmacol. Exp. Ther., 373, 438–444 (2020).
• 13) Matsumoto K, Kawanaka H, Hori M, Kusamori K, Utsumi D, Tsukahara T, Amagase K, Horie S, Yamamoto A, Ozaki H, Mori Y, Kato S. Role of transient receptor potential melastatin 2 in surgical inflammation and dysmotility in a mouse model of postoperative ileus. Am. J. Physiol. Gastrointest. Liver Physiol., 315, G104–G116 (2018).
• 14) Hayakawa T, Kase Y, Saito K, Hashimoto K, Ishige A, Komatsu Y, Sasaki H. Effect of Dai-Kenchu-to on intestinal obstruction following laparotomy. J. Smooth Muscle Res., 35, 47–54 (1999).
• 15) Tokita Y, Yuzurihara M, Satoh K, Iizuka S, Imamura S, Kase Y, Takeda S. The cholinergic nervous system plays an important role in rat postoperative intestinal adhesion. Surgery, 143, 226–232 (2008).
• 16) Ohnishi S, Fukumura K, Kubota R, Wajima T. Absorption, distribution, metabolism, and excretion of radiolabeled naldemedine in healthy subjects. Xenobiotica, 49, 1044–1053 (2019).
• 17) Kanemasa T, Koike K, Arai T, Ono H, Horita N, Chiba H, Nakamura A, Morioka Y, Kihara T, Hasegawa M. Pharmacologic effects of naldemedine, a peripherally acting µ-opioid receptor antagonist, in in vitro and in vivo models of opioid-induced constipation. Neurogastroenterol. Motil., 31, e13563 (2019).
• 18) Bauer AJ, Boeckxstaens GE. Mechanisms of postoperative ileus. Neurogastroenterol. Motil., 16 (Suppl. 2), 54–60 (2004).
• 19) Kraft MD. Emerging pharmacologic options for treating postoperative ileus. Am. J. Health Syst. Pharm., 64 (Suppl. 13), S13–S20 (2007).
• 20) Kraft M, Maclaren R, Du W, Owens G. Alvimopan (entereg) for the management of postoperative ileus in patients undergoing bowel resection. P. T., 35, 44–49 (2010).
• 21) Long DD, Armstrong SR, Beattie DT, Campbell CB, Church TJ, Colson PJ, Dalziel SM, Jacobsen JR, Jiang L, Obedencio GP, Rapta M, Saito D, Stergiades I, Tsuruda PR, Van Dyke PM, Vickery RG. Discovery of axelopran (TD-1211): a peripherally restricted μ-opioid receptor antagonist. ACS. Med. Chem. Lett., 10, 1641–1647 (2019).
• 22) Johnston IN, Milligan ED, Wieseler-Frank J, Frank MG, Zapata V, Campisi J, Langer S, Martin D, Green P, Fleshner M, Leinwand L, Maier SF, Watkins LR. A role for proinflammatory cytokines and fractalkine in analgesia, tolerance, and subsequent pain facilitation induced by chronic intrathecal morphine. J. Neurosci., 24, 7353–7365 (2004).
• 23) Yoshida S, Ohta J, Yamasaki K, Kamei H, Harada Y, Yahara T, Kaibara A, Ozaki K, Tajiri T, Shirouzu K. Effect of surgical stress on endogenous morphine and cytokine levels in the plasma after laparoscopoic or open cholecystectomy. Surg. Endosc., 14, 137–140 (2000).
• 24) Madbouly KM, Senagore AJ, Delaney CP. Endogenous morphine levels after laparoscopic versus open colectomy. Br. J. Surg., 97, 759–764 (2010).
• 25) Neary P, Delaney CP. Alvimopan. Expert Opin. Investig. Drugs, 14, 479–488 (2005).
• 26) Galligan JJ, Sternini C. Insights into the role of opioid receptors in the GI tract: experimental evidence and therapeutic relevance. Handb. Exp. Pharmacol., 239, 363–378 (2017).
• 27) Schmith VD, Garnett W, Barr WH, Kelleher D, Young M, Sanderlin G, Coots S, Agyemang A, Dukes G. Alvimopan pharmacokinetics (PK) & pharmacodynamics (PD) in patients with chronic constipation (CC). Clin. Pharmacol. Ther., 77, P49 (2005).
• 28) Beattie DT, Cheruvu M, Mai N, O’Keefe M, Johnson-Rabidoux S, Peterson C, Kaufman E, Vickery R. The in vitro pharmacology of the peripherally restricted opioid receptor antagonists, alvimopan, ADL 08-0011 and methylnaltrexone. Naunyn Schmiedebergs Arch. Pharmacol., 375, 205–220 (2020).
• 29) Berger NG, Ridolfi TJ, Ludwig KA. Delayed gastrointestinal recovery after abdominal operation—role of alvimopan. Clin. Exp. Gastroenterol., 8, 231–235 (2015).
• 30) Poston S, Broder MS, Gibbons MM, MacLaren R, Chang E, VandePol CJ, Cook SF, Techner L. Impact of alvimopan (entereg) on hospital costs after bowel resection: results from a large inpatient database. P T., 36, 209–220 (2011).