Antiemetic Effect of a Potent and Selective Neurokinin-1 Receptor Antagonist, FK886, on Cisplatin-Induced Acute and Delayed Emesis in Ferrets

Abstract

The antiemetic effect of a potent and selective neurokinin-1 (NK₁) receptor antagonist, FK886 ([3,5-bis(trifluoromethyl)phenyl][(2R)-2-(3-hydroxy-4-methylbenzyl)-4-{2-[(2S)-2-(methoxymethyl)morpholin-4-yl]ethyl}piperazin-1-yl]methanone dihydrochloride), on cisplatin-induced acute and delayed emesis in ferrets was studied. Intravenous administration of FK886 dose-dependently inhibited cisplatin (10 mg/kg)-induced acute emesis with a minimum effective dose (MED) of 0.32 mg/kg. In the same study, oral FK886 administered 8 h prior to cisplatin also dose-dependently inhibited the acute emesis during the 4-h observation period with an MED of 3.2 mg/kg. Further, when given by repeated oral administration of ≥1.6 mg/kg at 12-h intervals, the first dose being administered 1 min before cisplatin, FK886 significantly decreased the number of emetic responses in cisplatin (5 mg/kg)-induced delayed emesis. In the same study, oral FK886 (3.2 mg/kg) repeatedly administrated at 12-h intervals, the first dose being administered 36 h post cisplatin, also significantly attenuated the delayed emesis. Pharmacokinetic data in ferrets showed that plasma FK886 reached a maximum concentration within 0.5 h of administration, suggesting rapid oral absorption. In addition, rapid brain penetration of FK886 was suggested by complete and near complete inhibition of GR73632- and copper sulfate-induced emesis, respectively, by low-dose intravenous FK886 administered shortly before the emetogens. These results suggest that FK886 is an orally available NK₁ receptor antagonist which is effective against both the acute and delayed emesis induced by cisplatin. Because of its therapeutic efficacy on the delayed emesis and rapid brain distribution after oral administration, FK886 may have potential as an antiemetic agent that can be used for interventional treatment of chemotherapy-induced delayed emesis.

Nausea and vomiting are distressing symptoms that significantly detract from a patient’s overall quality of life and their recovery during medical treatment. Nausea and vomiting both occur as symptoms of many diseases and as side effects of medical treatments, including cancer chemotherapy, radiotherapy, and general anesthesia in surgery.¹⁾

Chemotherapy-induced nausea and vomiting (CINV) is perceived to be a major adverse effect of treatment by patients.²⁾ CINV may occur within hours of chemotherapy (acute phase) or be delayed until after the first 24 h, and then persist for several days (delayed phase). During the acute phase, chemotherapeutic agents induce serotonin (5-HT) release from enterochromaffin cells which stimulates local 5-HT₃ receptors on gastrointestinal vagal afferent nerves to initiate the vomiting reflex.^3–5) This may explain why 5-HT₃ antagonists have been shown to sufficiently control chemotherapy-induced acute emesis.^6–8) In contrast, 5-HT₃ antagonists are poorly effective against delayed emesis,^9,10) and the underlying mechanism of chemotherapy-induced delayed emesis has not been clearly identified.

Recently, neurokinin-1 (NK₁) receptor antagonists that penetrate the blood–brain barrier have been observed to block both the acute and delayed emesis induced by cisplatin in experimental animals,¹¹⁾ indicating that NK₁ receptor activation is involved in the central pathway leading to both emetic reflexes. Both the dorsal vagal complex, which includes the area postrema (AP), nucleus tractus solitarii (NTS) and dorsal motor nucleus of the vagus nerve (DMN), and the central pattern generator area in the brainstem have been proposed as key mediators of the emetic reflex, and are known to be heavily innervated by NK₁ receptor-expressing fibers.^12,13) Various emetic stimuli have been reported to increase the neuronal activity of this area in various species.^14–16) In particular, cisplatin causes an over-expression of NK₁ receptor proteins in the brainstem, which is closely associated with the peak acute- and delayed-phase vomiting frequencies in least shrews (Cryptotis parva).¹⁷⁾

Recently we have reported the pharmacological properties of a novel NK₁ receptor antagonist, FK886 ([3,5-bis(trifluoromethyl)phenyl][(2R)-2-(3-hydroxy-4-methylbenzyl)-4-{2-[(2S)-2-(methoxymethyl)morpholin-4-yl]ethyl}piperazin-1-yl]methanone dihydrochloride), which is a potent and selective NK₁ receptor antagonist that efficiently crosses the blood–brain barrier.¹⁸⁾ We also observed that the intravenous (i.v.) or oral (per os (p.o.)) administration of FK886 exerted strong antiemetic activity against cisplatin- and apomorphine-induced emetic responses in dogs.¹⁹⁾

Ferret cisplatin emesis models have been used extensively to evaluate clinically used antiemetics.^6,20) A systematic review and meta-analysis that assessed the translational validity of these ferret emesis models reported that the 10 mg/kg cisplatin model is appropriate to studying the acute phase of cisplatin-induced emesis, and that whereas the 5 mg/kg cisplatin model is not particularly valid as an acute emesis model, it is the most frequently used model to study cisplatin-induced delayed emesis.²¹⁾ In this study, we therefore evaluated the antiemetic efficacy of FK886 on cisplatin-induced acute and delayed emesis using these ferret cisplatin models.

MATERIALS AND METHODS

Animals

Male ferrets weighing 0.9–1.5 kg were purchased from Marshall Farms (North Rose, NY, U.S.A.). The ferrets were individually housed in a temperature- and humidity-controlled room with a 12-h light-dark cycle (light period from 07 : 00–19 : 00 h), and had a standard ferret diet (PMI Feeds, St. Louis, MO, U.S.A.) and water available ad libitum. The ferrets were allowed one week to acclimatize and were then transferred to observation cages in a quiet room and treated with emetogens. Emesis was characterized by the presence of rhythmic abdominal contractions that were associated with or without the oral expulsion of material from the gastrointestinal tract (vomiting or retching). In ferrets completely protected from emetic responses, the latency period was taken as equal to the observation period. All animal experimental procedures were approved by the Institutional Animal Care and Use Committee of Astellas Pharma Inc.

Cisplatin-Induced Acute Emesis

Ferrets were injected with cisplatin (10 mg/kg, i.v.) to evoke emesis and their behavior was recorded by a video camera for subsequent analysis.²²⁾ The number of emetic episodes (retching and vomiting) during the 4-h observation period and the latency to the first emetic episode were determined. To assess the antiemetic effect of FK886 on acute emesis in ferrets, FK886 (0.1–1 mg/kg, i.v.) or vehicle was administered 5 min before cisplatin and the minimum effective dose (MED) was determined. In another group, the effect of a 5-HT₃ antagonist, granisetron (0.01–0.1 mg/kg, i.v.), was also studied using the same procedures. The effect of oral administration of FK886 was studied to determine the MED to inhibit the acute emesis for 12 h. In this study, FK886 (0.32–3.2 mg/kg) or vehicle was administered 8 h before cisplatin and observation was for 4 h. Doses that completely protected two or more dogs in a group from emetic responses or significantly reduced the number of emetic episodes were regarded as effective.

Cisplatin-Induced Delayed Emesis

Ferrets were intraperitoneally (i.p.) injected with cisplatin (5 mg/kg) at 08 : 30 h and their behavior was recorded with a video camera equipped with automatic night vision for 3 d.²²⁾ During the experiments, ferrets were supplied with food (70 g/day) and water ad libitum. In prophylactic protocol experiments, FK886 (1.6 and 5 mg/kg, p.o.) or vehicle was administered 1 min before cisplatin and then every 12 h for 3 d. The behavior of the ferrets was assessed for the number of emetic episodes until 72 h after cisplatin. The emetic episodes during the 24–72 h observation period were defined as delayed emesis. In therapeutic protocol experiments, FK886 (3.2 mg/kg, p.o.) or vehicle was administered 36 h after cisplatin and continued every 12 h thereafter. The animal behavior was assessed for the number of emetic episodes during the 36–72 h observation period. In another group, the effect of granisetron was also studied. As the duration of action of granisetron at 3.2 mg/kg on cisplatin-induced acute emesis was reported to be at least 8 h,²³⁾ granisetron (3.2 mg/kg, i.v.) or vehicle was administered every 8 h from 32 h after cisplatin. The behavior of the ferrets was assessed for the number of emetic episodes during the 32–72 h observation period.

GR73632-Induced Emesis

Antiemetic activity of peripherally administered FK886 on emesis evoked by direct stimulation of central NK₁ receptors was studied to evaluate the ability of FK886 to cross the blood–brain barrier in ferrets. Under pentobarbital anesthesia (35 mg/kg, i.p.), a small hole was made in the skull at 2 mm anterior to the posterior edge of the skull in the midline. A stainless steel guide cannula (23-gauge, 13 mm in length) was inserted vertically 9 mm below the skull surface and was fixed to the skull with dental cement and two holding screws.²²⁾ Following the intramuscular (i.m.) administration of the antibiotic Cefamezin (Astellas Pharma Inc., Tokyo, Japan; 50 mg), the ferrets were returned to holding pens and allowed to recover for at least one week. A preliminary experiment to test the patency and placement of the cannula was conducted before test drug experiments. To evoke emesis, a specific NK₁ agonist, GR73632 (0.1 µg/head), was injected intracerebroventricularly (i.c.v.) into the fourth ventricle over a period of 2 min via the guide cannula using a 29-gauge needle that extended 7 mm beyond the tip of the guide cannula. Thirty seconds after GR73632 administration, the cannula was removed and the ferrets were transferred to a cage were they were observed continuously for 60 min for the time to onset of emetic episodes (retching or vomiting) and for the number of emetic episodes. Ferrets that exhibited no emetic episodes were omitted from the study. Following one week of recovery, the antiemetic effect of FK886 in the ferrets was studied. The ferrets were divided into two groups according to the number of emetic episodes in the preliminary experiment and FK886 (0.1 mg/kg, i.v.) or vehicle was administered 1 min before GR73632. Emesis was induced using the same procedure as that for the preliminary experiment and the latency and the number of emetic episodes were determined over a 60-min observation period. Cannula placements were then verified via an injection of Evans blue dye followed by a postmortem and sectioning through the brain.

Copper Sulfate-Induced Emesis

The brain penetration of FK886 in ferrets was further confirmed by a combination of peripheral administration of FK886 and copper sulfate-induced emesis. Following overnight fasting, the ferrets received a p.o. administration of copper sulfate at a dose of 40 mg/kg.²⁴⁾ FK886 (0.032–0.32 mg/kg, i.v.) or vehicle was administered 5 min before copper sulfate. In another group, the effect of granisetron was investigated as it has been reported to be ineffective against copper sulfate-induced emesis.²³⁾ In this group, ferrets received an i.v. administration of granisetron at a dose of 3.2 mg/kg, which is approximately 30 times higher than the dose that completely inhibits cisplatin-induced acute emesis, 5 min before copper sulfate. The ferrets were observed continuously for the time of onset of the emetic episodes (retching or vomiting) and the number of emetic episodes until 30 min after the copper sulfate administration.

Pharmacokinetics of FK886 in Ferrets

The ferrets received an i.v. or p.o. administration of FK886 at a dose of 3.2 mg/kg (n=3) and plasma samples were assayed for the parent drug after liquid–liquid extraction. A reverse-phase high performance liquid chromatography with tandem mass spectrometry (HPLC/MS/MS) method was used to determine FK886, with a lower limit of quantification of 1.0 ng/mL. Pharmacokinetic parameters were calculated by non-compartmental analysis using the MOMENT function in Microsoft Excel.²⁵⁾

Chemicals

FK886 and granisetron hydrochloride were synthesized at the Chemistry Research Laboratories of Astellas Pharma Inc. Cisplatin, GR73632 and copper sulfate were purchased from Sigma-Aldrich (St. Louis, MO, U.S.A.), Research Biochemicals International (Natick, MA, U.S.A.) and Wako Pure Chemical Industries, Ltd. (Tokyo, Japan), respectively. For i.v. administration, FK886 or granisetron was dissolved in 0.9% saline and administered in a volume of 0.25 mL/kg. For p.o. administration, FK886 or copper sulfate was dissolved in distilled water and administered in a volume of 2 mL/kg. For i.c.v. administration, GR73632 was dissolved in 0.9% saline and injected in a volume of 2 µL/head. Cisplatin was prepared in prewarmed (70°C) 0.9% saline followed by gradual cooling to 40°C and administered immediately. For i.v. or i.p. administration, cisplatin was made in a volume of 2 mL/kg or 5 mL/kg, respectively.

Data Analysis

Data were expressed as the mean±the standard error of the mean (S.E.M.) unless otherwise noted. Statistically significant differences in the latency and number of emetic episodes between control and compound-treated animals were determined by one-way analysis of variance followed by Dunnett’s multiple comparison tests. p-Values less than 0.05 were regarded as significant.

RESULTS

Effect on Cisplatin-Induced Acute Emesis

The administration of cisplatin at a dose of 10 mg/kg (i.v.) induced characteristic emetic responses in ferrets. The responses occurred within 80.6±5.4 (range 56.1–102.4) min and included 115.9±21.6 (range 17–302) emetic episodes during the 4-h observation period in vehicle-control animals (n=12). FK886 (0.1–1 mg/kg, i.v.) 5 min before cisplatin decreased the number of emetic episodes dose-dependently, and was significant at doses of 0.32 mg/kg and higher (Table 1). One of the four ferrets treated at a dose of 1 mg/kg was completely protected from emesis. The MED was determined as 0.32 mg/kg. Because FK886 has good oral availability in dogs,¹⁹⁾ we decided to conduct the delayed emesis study by p.o. administration. To set the dose for the delayed emesis study, the MED of orally administered FK886 to inhibit the acute emesis for 12 h was then determined. FK886 (0.32–3.2 mg/kg) administered 8 h before cisplatin dose-dependently decreased the number of emetic episodes during the 4-h observation period. Although there was no statistically significant difference in the number of emetic episodes observed between the control and FK886-treated animals, two of the four ferrets treated with 3.2 mg/kg were completely protected from emesis. Thus, the MED was determined as 3.2 mg/kg. Neither the intravenous nor oral administration of FK886 affected the latency to the first emetic episode in ferrets that were not completely protected from emesis.

Table 1. Effect of FK886 or Granisetron on Cisplatin-Induced Acute Emesis in Ferrets

Treatment (mg/kg)	Protected/tested	Retching+Vomiting		Latency (min)
		Number	% Inhibition
K886, i.v.
0	0/4	92.5±13.9	0	83.6±9.4
0.1	0/4	83.3±11.7	10	90.6±15.9
0.32	0/4	34.5±15.2*	62	95.5±10.4
1	1/4	6.0±3.5**	94	149.6±30.9
FK886, p.o.
0	0/4	119.5±39.1	0	69.8±10.5
0.32	0/4	134.5±35.9	−13	67.2±10.0
1	1/4	75.8±44.2	37	117.1±42.1
3.2	2/4	21.8±12.6	82	158.1±47.3
Granisetron, i.v.
0	0/4	135.8±55.8	0	88.5±7.4
0.01	1/4	71.3±26.9	48	152.3±39.5
0.032	1/4	62.0±26.4	54	137.1±34.8
0.1	4/4	0±0*	100	240±0**

Values are mean±S.E.M. * p<0.05, ** p<0.01 vs. control group as calculated by Dunnett’s multiple comparison test. Drugs were administered intravenously 5 min before cisplatin (10 mg/kg, i.v.), or orally 8 h before cisplatin (10 mg/kg, i.v.). The number of emetic episodes and latency to the first emetic episode were determined over a 4-h observation period following cisplatin administration.

Granisetron (0.01–0.1 mg/kg, i.v.), a 5-HT₃ receptor antagonist, dose-dependently and significantly decreased the number of emetic episodes (Table 1). One of the four ferrets treated with 0.01 or 0.032 mg/kg and all ferrets treated with 0.1 mg/kg were completely protected from emesis.

Effect on Cisplatin-Induced Delayed Emesis (Prophylactic Protocols)

The administration of cisplatin at a dose of 5 mg/kg induced emesis that peaked after approximately 48 h and lasted for 3 d. The number of emetic episodes in the vehicle-control animals was 53.3±31.4 (range 0–144) in the acute (0–24 h) phase and 792.5±134.0 (range 452–1055) in the delayed (24–72 h) phase (n=4). FK886 (1.6 and 5 mg/kg, p.o.) dose-dependently decreased the number of emetic episodes in acute and delayed phases (Fig. 1). The total number of emetic episodes during the delayed phase was significantly reduced at doses of 1.6 and 5 mg/kg (Table 2), with a dose of 5 mg/kg completely protecting one of the four ferrets from emetic responses.

Fig. 1. Effect of Oral FK886 on Cisplatin-Induced Delayed Emesis in a Prophylactic Protocol in Ferrets

Vehicle (A) or FK886 at 1.6 (B) or 5 mg/kg (C) was administered orally 1 min before and 12, 24, 36, 48 and 60 h after cisplatin (5 mg/kg, i.p.). Arrows indicate the administration of vehicle or FK886. Results are shown as the mean±S.E.M. (n=4) of emetic episodes in 4-h time bins.

Fig. 2. Effect of Oral FK886 on Cisplatin-Induced Delayed Emesis in a Therapeutic Protocol in Ferrets

Vehicle (A) or FK886 (3.2 mg/kg; B) was administered orally 36, 48, and 60 h after cisplatin (5 mg/kg, i.p.). Arrows indicate the administration of vehicle or FK886. Results are shown as the mean±S.E.M. (n=4) of emetic episodes in 4-h time bins.

Table 2. Effect of FK886 on Cisplatin-Induced Delayed Emesis in Ferrets (Prophylactic Protocol)

FK886 (mg/kg, p.o.)	Protected/tested	Retching+Vomiting
		Number	% Inhibition
0	0/4	792.5±134.0	0
1.6	0/4	165.7±6.2**	79
5	1/4	68.3±30.9***	91

Values are mean±S.E.M. ** p<0.01, *** p<0.001 vs. control group as calculated by Dunnett’s multiple comparison test. FK886 was administered orally 1 min before and 12, 24, 36, 48 and 60 h after cisplatin (5 mg/kg, i.p.). The number of emetic episodes during the delayed phase (24–72 h following cisplatin administration) was determined.

Effect on Cisplatin-Induced Delayed Emesis (Therapeutic Protocols)

The direct effect of FK886 on delayed emesis was evaluated by therapeutic protocols. The administration of cisplatin at a dose of 5 mg/kg reproducibly developed delayed emesis that included 361.5±59.6 (range 26–552) emetic episodes during the delayed phase (24–72 h after cisplatin) in vehicle-control animals (n=8). The repeated administration of FK886 at a dose of 3.2 mg/kg (p.o.) markedly inhibited the delayed emesis by about 80% of that in corresponding vehicle-control animals (Fig. 2, Table 3). In contrast, the repeated administration of granisetron at 3.2 mg/kg (i.v.) moderately, but not significantly, inhibited established emesis (Table 3).

Table 3. Effect of FK886 or Granisetron on Cisplatin-Induced Delayed Emesis in Ferrets (Therapeutic Protocol)

Treatment (mg/kg)	Protected/tested	Retching+Vomiting
		Number	% Inhibition
FK886, p.o.
0	0/4	319.3±71.5	0
3.2	0/4	68.0±22.5*	79
Granisetron, i.v.
0	0/4	268.8±92.4	0
3.2	0/4	109.0±69.9	59

Values are mean±S.E.M. * p<0.05 vs. control group as calculated by Student’s t-test. FK886 was administered orally 36, 48, and 60 h after cisplatin (5 mg/kg, i.p.). Granisetron was administered intravenously 32, 40, 48, 56 and 64 h after cisplatin (5 mg/kg, i.p.). The number of emetic episodes during the observation periods of 36–72 h or 32–72 h post cisplatin was determined for FK886 or granisetron, respectively.

Effect on GR73632-Induced Emesis

The direct stimulation of central NK₁ receptors by GR73632 (0.1 µg/head, i.c.v.) elicited emesis that was characterized by 74.4±54.5 (range 4–287) episodes of retching and vomiting within 1.4±0.6 (range 0.5–3.6) min of latency in the vehicle-control group (n=5). FK886 (0.1 mg/kg, i.v.) administered 1 min before GR73632 completely blocked emesis in all ferrets (n=5; Table 4).

Table 4. Effect of FK886 on GR73632-Induced Emesis in Ferrets

FK886 (mg/kg, i.v.)	Protected/tested	Retching+Vomiting		Latency (min)
		Number	% Inhibition
0	0/5	74.4±54.5	0	1.4±0.6
0.1	5/5	0±0	100	60±0***

Values are mean±S.E.M. *** p<0.001 vs. control group as calculated by Student’s t-test. FK886 was administered intravenously 1 min before GR73632 (0.1 µg/head, i.c.v.). The number of emetic episodes and latency to the first emetic response were determined for a 60-min observation period following GR73632 administration.

Effect on Copper Sulfate-Induced Emesis

Copper sulfate (40 mg/kg, p.o.) induced emesis that was characterized by 84.7±8.8 (range 49–115) episodes of retching and vomiting within 2.0±0.5 (range 0.2–3.2) min of latency in the vehicle-control group (n=7). FK886 (0.032–0.32 mg/kg, i.v.) administered 5 min before copper sulfate dose-dependently and significantly decreased the number of emetic episodes (Table 5), with two of the four ferrets in the 0.32 mg/kg dosing group being completely protected from emesis. In contrast, granisetron at a dose of 3.2 mg/kg (i.v.) had no effect on copper sulfate-induced emesis (Table 5).

Table 5. Effect of FK886 or Granisetron on Copper Sulfate-Induced Emesis in Ferrets

Treatment (mg/kg)	Protected/tested	Retching+Vomiting		Latency (min)
		Number	% Inhibition
FK886, i.v.
0	0/4	97.5±9.8	0	1.3±0.6
0.032	0/4	61.8±9.4*	36	2.4±0.5
0.1	0/4	18.3±9.8**	81	6.5±2.8
0.32	2/4	8.8±7.8**	91	17.0±7.5*
Granisetron, i.v.
0	0/3	67.7±9.4	0	2.9±0.3
3.2	0/3	81.7±6.2	−21	3.4±0.2

Values are mean±S.E.M. * p<0.05, ** p<0.01 vs. control group as calculated by Dunnett’s multiple comparison test. Drugs were administered intravenously 5 min before copper sulfate (40 mg/kg, p.o.). The number of emetic episodes and latency to the first emetic response were determined for a 30-min observation period following oral administration of copper sulfate.

Pharmacokinetics of FK886 in Ferrets

The plasma concentrations of FK886 after i.v. or p.o. administration at a dose of 3.2 mg/kg are shown in Fig. 3. After the i.v. administration of FK886, the plasma concentration declined in a bi-phasic fashion. The pharmacokinetic parameters of FK886 were evaluated by non-compartmental analysis according to the trapezoidal rule (Table 6). The terminal t_1/2 was 2.0 h, and total body clearance was 1.290 L/h/kg. The p.o. administration of FK886 resulted in rapid absorption and a maximum concentration (C_max) of 422 ng/mL was reached at the earliest sampling time of 0.5 h. The terminal t_1/2 was 1.9 h and bioavailability was 47%.

Fig. 3. Plasma Concentrations of FK886 after Intravenous (Closed Circles) or Oral (Open Circles) Administration at a Dose of 3.2 mg/kg

Each value represents the mean±S.D. (n=3).

Table 6. Pharmacokinetic Parameters after Single Intravenous or Oral Administration of FK886 at 3.2 mg/kg to Male Ferrets

Route	C0 (ng/mL)	Cmax (ng/mL)	Tmax (h)	t1/2 (h)	AUC (ng·h/mL)	CLtot (L/h/kg)	CL/F (L/h/kg)	F
i.v.	2275±190	—	—	2.0±0.3	2098±165	1.290 ±0.102	—	—
p.o.	—	422±116	0.5±0.0	1.9±0.5	985±168	—	2.784 ±0.510	47

Values are mean±S.D. (n=3)

DISCUSSION

In the present study, FK886 demonstrated high antiemetic efficacy on cisplatin-induced acute and delayed emesis in ferrets. We employed the 10 mg/kg and 5 mg/kg cisplatin models for acute and delayed emesis, respectively.^21,22) It has been reported that 5-HT₃ receptor antagonists completely abolish cisplatin-induced acute emesis, but only modestly inhibit the delayed emesis in experimental animal models.⁹⁾ We have thus verified the efficacy of 5-HT₃ antagonist against cisplatin-induced acute and delayed emesis using granisetron and consider these models as an accurate representation of chemotherapy-induced emesis-like responses in humans.

The repeated oral administration of FK886 at doses of 1.6 mg/kg or more at 12-h intervals in a prophylactic manner significantly reduced the delayed emesis. Further, repeated oral administration of FK886 at a dose of 3.2 mg/kg at 12-h intervals in a therapeutic manner also significantly attenuated the delayed emesis. The MED of orally administered FK886 on acute emesis was determined as 3.2 mg/kg. Thus, the potency of FK886 on cisplatin-induced delayed emesis looked closely similar to that on acute emesis irrespective of dosing protocol, suggesting that FK886 may attenuate both acute and delayed emesis in a direct manner.

Interestingly, the strongest inhibition of emesis was observed in the first 4-h bin after each oral administration of FK886 (Figs. 1(B), (C), Fig. 2(B)). Pharmacokinetics revealed the rapid absorption of orally administered FK886, which reached a maximum plasma concentration within 30 min of administration. In addition, the rapid brain penetration of FK886 was suggested in GR73632- and copper sulfate-induced emesis studies. Copper sulfate has been reported to directly stimulate the gastric luminal chemoreceptors, whose signals are relayed via the vagal afferent nerves to the brainstem to induce emesis.³⁾ The latency to the first emetic episode in this model is therefore extremely short (<3 min) and is almost comparable to that in emesis caused by direct stimulation of central NK₁ receptors. The strong inhibition of emetic episodes in those models by FK886, which was dosed i.v. 1 min before GR73632 or 5 min before copper sulfate, suggests a rapid and efficient distribution of FK886 into the brain. Taken together, orally administered FK886 may be absorbed and distributed to the brain quickly and exert an immediate antiemetic activity against delayed emesis in the ferrets.

CINV has a major adverse impact on patients undergoing chemotherapy.²⁾ CINV can have a multitude of consequences for patients, limited not only to physical damage caused by impaired appetite and/or fluid intake, but also psychological distress that often leads to a patient’s refusal to continue with the chemotherapy or anticipatory emesis in the subsequent cycle of chemotherapy.^26,27) Although antiemetic regimens that include 5-HT₃ receptor antagonist administration provide better control of acute emesis, these have been less effective in improving delayed emesis.^10,28) It was reported that delayed emesis remains a significant cause of treatment-related morbidity in 40–75% of cancer patients despite the use of various antiemetic drugs.²⁷⁾ Therefore, better antiemetic regimens are still needed for the control of delayed emesis. It is important to consider, however, that not all patients who receive chemotherapy experience delayed emesis. For example, the incidence of delayed emesis associated with cisplatin therapy has been reported to range from 43 to 89%.^28,29) Further, the prediction of delayed emesis is difficult as contributing factors to its development include sex, age, dose of chemotherapy, and control of acute emesis.³⁰⁾ Taking these factors into consideration, antiemetics that can be administered after the first emetic episode might be useful in avoiding unnecessary treatment of delayed emesis. Our present and previous reports have demonstrated the excellent antiemetic efficacies of NK₁ receptor antagonists even when dosed after the onset of the delayed emesis.^20,31) This suggests the possibility of using NK₁ antagonists for the treatment of delayed emesis. Of note, however, the clinically available NK₁ receptor antagonist aprepitant is administered for the prevention of CINV but not for the treatment of nausea and vomiting, which is possibly due to its relatively slow distribution to the brain.^32,33) Here, we showed the immediate antiemetic activity of the oral administration of FK886 against cisplatin-induced delayed emesis. This is consistent with previous reports of the rapid and efficient distribution of FK886 into the brain in several species other than ferrets.^18,19)

Taken together, these findings indicate that FK886 might have therapeutic potential as an antiemetic agent that can be used for interventional treatment of delayed emesis and provide novel control of CINV in clinical settings.

Acknowledgment

The authors wish to thank Dr. Kenji Tabata and Dr. Toshio Teramura, Analysis & Pharmacokinetics Research Laboratories, Drug Discovery Research, Astellas Pharma Inc. for their patience and encouragement.

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