Mechanism of Action
Paliperidone palmitate is hydrolyzed to paliperidone [see Clinical Pharmacology]. Paliperidone is the major active metabolite of risperidone. The mechanism of action of paliperidone, as with other drugs having efficacy in schizophrenia, is unknown, but it has been proposed that the drug's therapeutic activity in schizophrenia is mediated through a combination of central dopamine Type 2 (D2) and serotonin Type 2 (5HT2A) receptor antagonism.
Paliperidone is a centrally active dopamine Type 2 (D2) receptor antagonist and a serotonin Type 2 (5HT2A) receptor antagonist. Paliperidone is also active as an antagonist at alpha1 and alpha2 adrenergic receptors and H1 histaminergic receptors, which may explain some of the other effects of the drug. Paliperidone has no affinity for cholinergic muscarinic or beta1- and beta2-adrenergic receptors. The pharmacological activity of the (+)- and (-)- paliperidone enantiomers is qualitatively and quantitatively similar in vitro.
Absorption and Distribution
Due to its extremely low water solubility, paliperidone palmitate dissolves slowly after intramuscular injection before being hydrolyzed to paliperidone and absorbed into the systemic circulation. Following a single intramuscular dose, the plasma concentrations of paliperidone gradually rise to reach maximum plasma concentrations at a median Tmax of 13 days. The release of the drug starts as early as day 1 and lasts for as long as 126 days.
Following intramuscular injection of single doses (39 mg 234 mg) in the deltoid muscle, on average, a 28% higher Cmax was observed compared with injection in the gluteal muscle. The two initial deltoid intramuscular injections of 234 mg on day 1 and 156 mg on day 8 help attain therapeutic concentrations rapidly. The release profile and dosing regimen of INVEGA® SUSTENNA® results in sustained therapeutic concentrations. The AUC of paliperidone following INVEGA® SUSTENNA® administration was dose-proportional over a 39 mg 234 mg dose range, and less than dose-proportional for Cmax for doses exceeding 78 mg. The mean steady-state peak:trough ratio for a INVEGA® SUSTENNA® dose of 156 mg was 1.8 following gluteal administration and 2.2 following deltoid administration.
Following administration of paliperidone palmitate the (+) and (-) enantiomers of paliperidone interconvert, reaching an AUC (+) to (-) ratio of approximately 1.61.8.
Based on a population analysis, the apparent volume of distribution of paliperidone is 391 L. The plasma protein binding of racemic paliperidone is 74%.
Metabolism and Elimination
In a study with oral immediate-release 14C-paliperidone, one week following administration of a single oral dose of 1 mg immediate-release 14C-paliperidone, 59% of the dose was excreted unchanged into urine, indicating that paliperidone is not extensively metabolized in the liver. Approximately 80% of the administered radioactivity was recovered in urine and 11% in the feces. Four metabolic pathways have been identified in vivo, none of which accounted for more than 10% of the dose: dealkylation, hydroxylation, dehydrogenation, and benzisoxazole scission. Although in vitro studies suggested a role for CYP2D6 and CYP3A4 in the metabolism of paliperidone, there is no evidence in vivo that these isozymes play a significant role in the metabolism of paliperidone. Population pharmacokinetics analyses indicated no discernable difference on the apparent clearance of paliperidone after administration of oral paliperidone between extensive metabolizers and poor metabolizers of CYP2D6 substrates. In vitro studies in human liver microsomes showed that paliperidone does not substantially inhibit the metabolism of medicines metabolized by cytochrome P450 isozymes, including CYP1A2, CYP2A6, CYP2C8/9/10, CYP2D6, CYP2E1, CYP3A4, and CYP3A5.
In vitro studies have shown that paliperidone is a P-gp substrate and a weak inhibitor of P-gp at high concentrations. No in vivo data are available and the clinical relevance is unknown.
The median apparent half-life of paliperidone following INVEGA® SUSTENNA® single-dose administration over the dose range of 39 mg 234 mg ranged from 25 days 49 days.
Long-Acting Paliperidone Palmitate Injection versus Oral Extended-Release Paliperidone
INVEGA® SUSTENNA® is designed to deliver paliperidone over a monthly period while extended-release oral paliperidone is administered on a daily basis. The initiation regimen for INVEGA® SUSTENNA® (234 mg/156 mg in the deltoid muscle on Day 1/Day 8) was designed to rapidly attain steady-state paliperidone concentrations when initiating therapy without the use of oral supplementation.
In general, overall initiation plasma levels with INVEGA® SUSTENNA® were within the exposure range observed with 612 mg extended-release oral paliperidone. The use of the INVEGA® SUSTENNA® initiation regimen allowed patients to stay in this exposure window of 612 mg extended-release oral paliperidone even on trough pre-dose days (Day 8 and Day 36). The intersubject variability for paliperidone pharmacokinetics following delivery from INVEGA® SUSTENNA® was lower relative to the variability determined from extended-release oral paliperidone tablets. Because of the difference in median pharmacokinetic profiles between the two products, caution should be exercised when making a direct comparison of their pharmacokinetic properties.
INVEGA® SUSTENNA® has not been systematically studied in patients with renal impairment. Based on a limited number of observations with INVEGA® SUSTENNA® in subjects with mild renal impairment and pharmacokinetic simulations, the dose of INVEGA® SUSTENNA® should be reduced in patients with mild renal impairment; INVEGA® SUSTENNA® is not recommended in patients with moderate or severe renal impairment [see Dosage and Administration]. Although INVEGA® SUSTENNA® was not studied in patients with moderate or severe renal impairment, the disposition of a single oral dose paliperidone 3 mg extended-release tablet was studied in subjects with varying degrees of renal function. Elimination of paliperidone decreased with decreasing estimated creatinine clearance. Total clearance of paliperidone was reduced in subjects with impaired renal function by 32% on average in mild (CrCl = 50 mL/min to < 80 mL/min), 64% in moderate (CrCl = 30 mL/min to < 50 mL/min), and 71% in severe (CrCl = 10 mL/min to < 30 mL/min) renal impairment, corresponding to an average increase in exposure (AUCinf) of 1.5 fold, 2.6 fold, and 4.8 fold, respectively, compared to healthy subjects. Based on a limited number of observations with INVEGA® SUSTENNA® in subjects with mild renal impairment and pharmacokinetic simulations, the recommended initiation of INVEGA® SUSTENNA® for patients with mild renal impairment is with a dose of 156 mg on treatment day 1 and 117 mg on treatment day 8; thereafter, follow with monthly injections of 78 mg [see Dosage and Administration].
INVEGA® SUSTENNA® has not been studied in patients with hepatic impairment. Based on a study with oral paliperidone in subjects with moderate hepatic impairment (Child-Pugh class B), no dose adjustment is required in patients with mild or moderate hepatic impairment [see Dosage and Administration]. In the study with oral paliperidone in subjects with moderate hepatic impairment (Child-Pugh class B), the plasma concentrations of free paliperidone were similar to those of healthy subjects, although total paliperidone exposure decreased because of a decrease in protein binding. Paliperidone has not been studied in patients with severe hepatic impairment.
No dosage adjustment is recommended based on age alone. However, dose adjustment may be required because of age-related decreases in creatinine clearance [see Renal Impairment above and Dosage and Administration].
No dosage adjustment is recommended based on race. No differences in pharmacokinetics were observed between Japanese and Caucasians.
No dosage adjustment is recommended based on gender, although slower absorption was observed in females in a population pharmacokinetic analysis.
No dosage adjustment is recommended based on smoking status. Based on in vitro studies utilizing human liver enzymes, paliperidone is not a substrate for CYP1A2; smoking should, therefore, not have an effect on the pharmacokinetics of paliperidone.
Carcinogenesis, Mutagenesis, Impairment of Fertility
The carcinogenic potential of intramuscularly injected paliperidone palmitate was assessed in rats. There was an increase in mammary gland adenocarcinomas in female rats at 16, 47, and 94 mg/kg/month, which is 0.6, 2, and 4 times, respectively, the maximum recommended human 234 mg dose of INVEGA® SUSTENNA® on a mg/m2 basis. A no-effect dose was not established. Male rats showed an increase in mammary gland adenomas, fibroadenomas, and carcinomas at 47 mg and 94 mg/kg/month. A carcinogenicity study in mice has not been conducted with paliperidone palmitate.
Carcinogenicity studies of risperidone, which is extensively converted to paliperidone in rats, mice, and humans, were conducted in Swiss albino mice and Wistar rats. Risperidone was administered in the diet at daily doses of 0.63, 2.5, and 10 mg/kg for 18 months to mice and for 25 months to rats. A maximum tolerated dose was not achieved in male mice. There were statistically significant increases in pituitary gland adenomas, endocrine pancreas adenomas, and mammary gland adenocarcinomas. The no-effect dose for these tumors was less than or equal to the maximum recommended human dose of risperidone on a mg/m2 basis (see RISPERDAL® package insert). An increase in mammary, pituitary, and endocrine pancreas neoplasms has been found in rodents after chronic administration of other antipsychotic drugs and is considered to be mediated by prolonged dopamine D2-receptor antagonism and hyperprolactinemia. The relevance of these tumor findings in rodents in terms of human risk is unknown [see Warnings and Precautions].
Paliperidone palmitate showed no genotoxic potential in the Ames reverse mutation test or the mouse lymphoma assay. No evidence of genotoxic potential for paliperidone was found in the Ames reverse mutation test, the mouse lymphoma assay, or the in vivo rat micronucleus test.
Impairment of Fertility
Fertility studies of paliperidone palmitate have not been performed.
In a study of fertility conducted with orally administered paliperidone, the percentage of treated female rats that became pregnant was not affected at doses of paliperidone of up to 2.5 mg/kg/day. However, pre- and post-implantation loss were increased, and the number of live embryos was slightly decreased, at 2.5 mg/kg, a dose that also caused slight maternal toxicity. These parameters were not affected at a dose of 0.63 mg/kg, which is half of the maximum recommended human dose (12 mg/day) of orally administered paliperidone (INVEGA®) on a mg/m2 basis.
The fertility of male rats was not affected at oral doses of paliperidone of up to 2.5 mg/kg/day, although sperm count and sperm viability studies were not conducted with paliperidone. In a subchronic study in Beagle dogs with risperidone, which is extensively converted to paliperidone in dogs and humans, all doses tested (0.31 mg/kg 5.0 mg/kg) resulted in decreases in serum testosterone and in sperm motility and concentration. Serum testosterone and sperm parameters partially recovered, but remained decreased after the last observation (two months after treatment was discontinued).