Pharmacology: Mechanism of Action:
Lubiprostone is a locally acting chloride channel activator that enhances a chloride-rich intestinal fluid secretion without altering sodium and potassium concentrations in the serum. Lubiprostone acts by specifically activating ClC-2, which is a normal constituent of the apical membrane of the human intestine, in a protein kinase A-independent fashion.
By increasing intestinal fluid secretion, lubiprostone increases motility in the intestine, thereby facilitating the passage of stool and alleviating symptoms associated with chronic idiopathic constipation. Patch clamp cell studies in human cell lines have indicated that the majority of the beneficial biological activity of lubiprostone and its metabolites is observed only on the apical (luminal) portion of the gastrointestinal epithelium.
Lubiprostone, via activation of apical ClC-2 channels in intestinal epithelial cells, bypasses the antisecretory action of opiates that results from suppression of secretomotor neuron excitability.
Activation of ClC-2 by lubiprostone has also been shown to stimulate recovery of mucosal barrier function and reduce intestinal permeability via the restoration of tight junction protein complexes in ex vivo
studies of ischemic porcine intestine.
Although the pharmacologic effects of lubiprostone in humans have not been fully evaluated, animal studies have shown that oral administration of lubiprostone increases chloride ion transport into the intestinal lumen, enhances fluid secretion into the bowels, and improves fecal transit.
Following oral administration concentrations of lubiprostone in plasma are below the level of quantitation (10 pg/mL). Therefore, standard pharmacokinetic parameters such as area under the curve (AUC), maximum concentration (Cmax
), and half-life (t½
) cannot be reliably calculated. However, the pharmacokinetic parameters of M3 (only measurable active metabolite of lubiprostone) have been characterized.
Peak plasma concentration of M3, after a single oral dose of 24 mcg of lubiprostone, occurred at approximately 1 hours. The Cmax
was 41.5 pg/mL and the mean AUC0-t
was 57.1 pg·hr/mL. The AUC0-t
of M3 increases dose proportionally after single 24-mcg and 144-mcg doses of lubiprostone (6-times the maximum recommended 24 mcg dose).
Food Effect: A study was conducted with a single 72-mcg dose of 3H-labeled lubiprostone (3-times the maximum recommended 24 mcg dose) to evaluate the potential of a food effect on lubiprostone absorption, metabolism, and excretion. Pharmacokinetic parameters of total radioactivity demonstrated that Cmax
decreased by 55% while AUC0-∞
was unchanged when lubiprostone was administered with a high-fat meal. The clinical relevance of the effect of food on the pharmacokinetics of lubiprostone is not clear. However, lubiprostone was administered with food and water in a majority of clinical trials.
Distribution: In vitro
protein binding studies indicate lubiprostone is approximately 94% bound to human plasma proteins.
Metabolism: Lubiprostone is rapidly and extensively metabolized by 15-position reduction, α-chain β-oxidation, and ω-chain ω-oxidation. In vitro
studies using human liver microsomes indicate that cytochrome P45 isoenzymes are not involved in the metabolism of lubiprostone. Further in vitro
studies indicate that M3, a metabolite of lubiprostone, is formed by the reduction of the 15-carbonyl moiety to a hydroxy moiety by microsomal carbonyl reductase. M3 makes up less than 10% of the dose of radiolabeled lubiprostone.
Animal studies have shown that metabolism of lubiprostone rapidly occurs within the stomach and jejunum, most likely in the absence of any systemic absorption.
Excretion: Lubiprostone could not be detected in plasma; however, M3 has a t½
ranging from 0.9 to 1.4 hours. After a single oral dose of 72 mcg of 3
H-labeled lubiprostone, 60% of total administered radioactivity was recovered in the urine within 24 hours and 30% of total administered radioactivity was recovered in the feces by 168 hours. Lubiprostone and M3 are only detected in trace amounts in human feces.
Male and Female Patients: The pharmacokinetics of M3 were similar between male and female subjects.
Patients with Renal Impairment: Sixteen subjects, 34 to 47 years old (8 severe renally impaired subjects [creatinine clearance (CrCl) less than 20 mL/min] who required hemodialysis and 8 control subjects with normal renal function [CrCl above 80 mL/min]), received a single oral 24-mcg dose of Amitiza. Following administration, lubiprostone plasma concentrations were below the limit of quantitation (10 pg/mL). Plasma concentrations of M3 were within the range of exposure from previous clinical experience with Amitiza.
Patients with Hepatic Impairment: Twenty-five subjects, 38 to 78 years old (9 with severe hepatic impairment [Child-Pugh Class C], 8 with moderate impairment [Child-Pugh Class B], and 8 with normal liver function), received either 12 mcg or 24 mcg of Amitiza under fasting conditions. Following administration, lubiprostone plasma concentrations were below the limit of quantitation (10 pg/mL) except for two subjects. In moderately and severely impaired subjects, the Cmax
of the active lubiprostone metabolite M3 were increased, as shown in Table 1. (See Table 1.)
Click on icon to see table/diagram/image
These results demonstrate that there is a correlation between increased exposure of M3 and severity of hepatic impairment [see Hepatic Impairment under Precautions].
Drug Interaction Studies: Based upon the results of in vitro
human microsome studies, there is low likelihood of pharmacokinetic drug-drug interactions with lubiprostone. Additionally, in vitro
studies in human liver microsomes demonstrate that lubiprostone does not inhibit cytochrome P450 isoforms 3A4, 2D6, 1A2, 2A6, 2B6, 2C9, 2C19, or 2E1, and in vitr
o studies of primary cultures of human hepatocytes show no induction of cytochrome P450 isoforms 1A2, 2B6, 2C9, and 3A4 by lubiprostone. Based on the available information, no protein binding-mediated drug interactions of clinical significance are anticipated.
Two 2-year oral (gavage) carcinogenicity studies (one in Crl:B6C3F mice and one in Sprague-Dawley rats) were conducted with lubiprostone. In the 2-year carcinogenicity study in mice, lubiprostone doses of 25, 75, 200, and 500 mcg/kg/day (approximately 2, 6, 17, and 42 times the maximum recommended human dose, respectively, based on body surface area (mg/m2)) were used. In the 2-year rat carcinogenicity study, lubiprostone doses of 20, 100, and 400 mcg/kg/day (approximately 3, 17, and 68 times the maximum recommended human dose, respectively, based on body surface area (mg/m2)) were used. In the mouse carcinogenicity study, there was no significant increase in any tumor incidences. There was a significant increase in the incidence of interstitial cell adenoma of the testes in male rats at the 400 mcg/kg/day dose. In female rats, treatment with lubiprostone produced hepatocellular adenoma at the 400 mcg/kg/day dose.
Lubiprostone was not genotoxic in the in vitro
Ames reverse mutation assay, the in vitro
mouse lymphoma (L5178Y TK+/-
) forward mutation assay, the in vitro
Chinese hamster lung (CHL/IU) chromosomal aberration assay, and the in vivo
mouse bone marrow micronucleus assay.
Impairment of Fertility:
Lubiprostone, at oral doses of up to 1000 mcg/kg/day, had no effect on the fertility and reproductive function of male and female rats. However, the number of implantation sites and live embryos were significantly reduced in rats at the 1000 mcg/kg/day dose as compared to control. The number of dead or resorbed embryos in the 1000 mcg/kg/day group was higher compared to the control group, but was not statistically significant. The 1000 mcg/kg/day dose in rats is approximately 169 times the maximum recommended human dose of 48 mcg/day, based on body surface area (mg/m2).
Chronic Idiopathic Constipation in Adults: Two double-blinded, placebo-controlled studies of identical design were conducted in patients with CIC. CIC was defined as, on average, less than 3 SBMs per week (a SBM is a bowel movement occurring in the absence of laxative use) along with one or more of the following symptoms of constipation for at least 6 months prior to randomization: very hard stools for at least a quarter of all bowel movements; sensation of incomplete evacuation following at least a quarter of all bowel movements; and straining with defecation at least a quarter of the time.
Following a 2-week baseline/washout period, a total of 479 patients (mean age 47 [range 20 to 81] years; 89% female; 81% Caucasian, 10% African American, 7% Hispanic, 2% Asian; 11% at least 65 years of age) were randomized and received Amitiza 24 mcg twice daily or placebo twice daily for 4 weeks. The primary endpoint of the studies was SBM frequency. The studies demonstrated that patients treated with Amitiza had a higher frequency of SBMs during Week 1 than the placebo patients. In both studies, results similar to those in Week 1 were also observed in Weeks 2, 3, and 4 of therapy (Table 2). (See Table 2.)
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In both studies, Amitiza demonstrated increases in the percentage of patients who experienced SBMs within the first 24 hours after administration when compared to placebo (57% vs. 37% in Study 1 and 63% vs. 32% in Study 2, respectively). Similarly, the time to first SBM was shorter for patients receiving Amitiza than for those receiving placebo.
Signs and symptoms related to constipation, including abdominal bloating, abdominal discomfort, stool consistency, and straining, as well as constipation severity ratings, were also improved with Amitiza versus placebo. The results were consistent in subpopulation analyses for gender, race, and elderly patients (at least 65 years of age).
During a 7-week randomized withdrawal study, patients who received Amitiza during a 4-week treatment period were then randomized to receive either placebo or to continue treatment with Amitiza. In Amitiza-treated patients randomized to placebo, SBM frequency rates returned toward baseline within 1 week and did not result in worsening compared to baseline. Patients who continued on Amitiza maintained their response to therapy over the additional 3 weeks of treatment.
Irritable Bowel Syndrome with Constipation in Adults: Two double-blinded, placebo-controlled studies of similar design were conducted in adult patients with IBS-C. IBS was defined as abdominal pain or discomfort occurring over at least 6 months with two or more of the following: relieved with defecation; onset associated with a change in stool frequency; and onset associated with a change in stool form. Patients were sub-typed as having IBS-C if they also experienced two of three of the following: 1) < 3 spontaneous bowel movements (SBMs) per week; > 25% hard stools, and > 25% SBMs associated with straining.
Following a 4-week baseline/washout period, a total of 1154 patients (mean age 47 [range 18 to 85] years; 92% female; 77% Caucasian, 13% African American, 9% Hispanic, 0.4% Asian; 8 % at least 65 years of age) were randomized and received Amitiza 8 mcg twice daily (16 mcg/day) or placebo twice daily for 12 weeks. The primary efficacy endpoint was assessed weekly utilizing the patient's response to a global symptom relief question based on a 7-point, balanced scale ("significantly worse" to "significantly relieved"): "How would you rate your relief of IBS symptoms (abdominal discomfort/pain, bowel habits, and other IBS symptoms) over the past week compared to how you felt before you entered the study?"
The primary efficacy analysis was a comparison of the proportion of "overall responders" in each arm. A patient was considered an "overall responder" if the criteria for being designated a "monthly responder" were met in at least 2 of the 3 months on study. A "monthly responder" was defined as a patient who had reported "significantly relieved" for at least 2 weeks of the month or at least "moderately relieved" in all 4 weeks of that month. During each monthly evaluation period, patients reporting "moderately worse" or "significantly worse" relief, an increase in rescue medication use, or those who discontinued due to lack of efficacy, were deemed non-responders.
The percentage of patients in Study 1 qualifying as an "overall responder" was 14% in the group receiving Amitiza 8 mcg twice daily compared to 7.8% of patients receiving placebo twice daily. In Study 2, 12.1% of patients in the Amitiza 8 mcg group were "overall responders" versus 6% of patients in the placebo group. In both studies, the treatment differences between the placebo and Amitiza groups were statistically significant.
Results in men: The two randomized, placebo-controlled, double-blinded studies comprised 97 (8%) male patients, which is insufficient to determine whether men with IBS-C respond differently to Amitiza from women.
During a 4-week randomized withdrawal period following Study 1, patients who received Amitiza during the 12-week treatment period were re-randomized to receive either placebo or to continue treatment with Amitiza. In Amitiza-treated patients who were "overall responders" during Study 1 and who were re-randomized to placebo, SBM frequency rates did not result in worsening compared to baseline.