Pharmacokinetics of ADS-5102 (Amantadine) Extended Release Capsules Administered Once Daily at Bedtime for the Treatment of Dyskinesia

BACKGROUND

Preclinical and clinical studies suggest amantadine immediate-release (IR) may reduce dyskinesia in Parkinson's disease (PD), although higher doses are associated with increased CNS adverse events (AEs). ADS-5102 is an extended release amantadine capsule formulation, designed for once-daily dosing at bedtime (qhs) to provide high concentrations upon waking and throughout the day, with lower concentrations in the evening. The pharmacokinetics (PK) of ADS-5102 were assessed in two phase I studies in healthy subjects, and a blinded, randomized phase II/III dose-finding study in PD patients.

METHODS

The first phase I study assessed single ADS-5102 doses (68.5, 137, and 274 mg) in a crossover design, whereas the second phase I study evaluated ADS-5102 137 mg for 7 days followed by amantadine IR 81 mg twice daily (or reverse order). In the phase II/III double-blind study, PD patients with dyskinesia were randomized to ADS-5102 (210, 274, or 338 mg) or placebo for 8 weeks.

RESULTS

Single ADS-5102 doses resulted in a slow initial rise in amantadine plasma concentration, with delayed time to maximum concentration (12-16 h). Amantadine plasma concentrations were higher in PD patients versus healthy volunteers. The steady-state profile of once-daily ADS-5102 was significantly different from that of twice-daily amantadine IR, such that the two formulations are not bioequivalent. PK modeling suggested the recommended daily ADS-5102 dosage (274 mg qhs) resulted in 1.4- to 2.0-fold higher amantadine plasma concentrations during the day versus amantadine IR.

CONCLUSIONS

ADS-5102 can be administered once-daily qhs to achieve high amantadine plasma concentrations in the morning and throughout the day, when symptoms of dyskinesia occur.

Amantadine, apomorphine and zolpidem in the treatment of disorders of consciousness

Survivors of severe brain injuries may end up in a state of 'wakeful unresponsiveness' or in a minimally conscious state. Pharmacological treatments of patients with disorders of consciousness aim to improve arousal levels and recovery of consciousness. We here provide a systematic overview of the therapeutic effects of amantadine, apomorphine and zolpidem in patients recovering from coma. Evidence from clinical trials using these commonly prescribed pharmacological agents suggests positive changes of the patients' neurological status, leading sometimes to dramatic improvements. These findings are discussed in the context of current hypotheses of these agents' therapeutic mechanisms on cerebral function. In order to improve our understanding of the underlying pathophysiological mechanisms of these drugs, we suggest combining sensitive and specific behavioral tools with neuroimaging and electrophysiological measures in large randomized, double-blind, placebo-controlled experimental designs. We conclude that the pharmacokinetics and pharmacodynamics of amantadine, apomorphine and zolpidem need further exploration to determine which treatment would provide a better neurological outcome regarding the patient's etiology, diagnosis, time since injury and overall condition.

[Corneal toxicity due to amantadine]

CASE REPORT

A 64 year-old female with Parkinson disease treated with amantadine for two years who suddenly suffered bilateral corneal oedema. It was initially treated as herpetic endotheliitis without improvement as we lacked information on her chronic treatment. The corneal oedema finally resolved after withdrawing the drug.

DISCUSSION

Amantadine hydrochloride may produce endothelial dysfunction. Once the amantadine treatment is stopped, the corneal oedema may be reversible but endothelial density remains low. An ophthalmologist examination should be performed before the initiation of amantadine treatment in order to establish a risk: benefit ratio, especially in those patients with low endothelial density or any endothelial anomaly.

Quantitative determination of amantadine in human plasma by liquid chromatography–mass spectrometry and the application in a bioequivalence study

A sensitive liquid chromatography-electrospray ionization mass spectrometry (LC-ESI-MS) method is developed and validated for rapid determination of amantadine in human plasma. Desloratadine was used as the internal standard (I.S.). Human plasma (0.2 mL) was first alkalified with 100 microL of sodium hydroxide (3M) and then extracted with 1 mL of n-hexane containing 1% isopropanol (v/v) and 10% dichloromethane (v/v) by vortex-mixer for 3 min. The mixture was centrifuged at 14,000 rpm for 5 min. The supernatant was evaporated to dryness and the residue was dissolved in mobile phase. Samples were separated using a Thermo Hypersil-HyPURITYC18 reversed-phase column (150 mm x 2.1 mm i.d., 5 microm). Mobile phase consisted of methanol-acetonitrile-20 mM ammonium acetate (45:10:45, v/v/v) containing 1% acetic acid with pH 4.0. Amantadine and I.S. were measured by electrospray ion source in positive selective ion monitoring mode. The good linearity ranged from 3.9 to 1000 ng/mL and the lowest limit of quantification was 3.9 ng/mL. The extraction efficiencies were approximately 70% and recoveries of method ranged from 98.53 to 103.24%. The intra-day relative standard deviations (R.S.D.) were less than 8.43% and inter-day R.S.D. below 10.59%. The quality control samples were stable when kept at room temperature for 12h, at -20 degrees C for 30 days and after four freeze/thaw cycles. The method has been successfully used to evaluation of the pharmacokinetics and bioequivalence of amantadine in 20 healthy volunteers after an oral dose of 100 mg amantadine.

Pharmacokinetic Characterization of Amantadine in Human Brain Tissue

Amantadine concentrations in human brain tissue were assessed in order to estimate population pharmacokinetic parameters using the computer program NONMEM. The elimination constant in brain tissue was determined to be 0.00447 [1/h], resulting in a t1/2 of 6.5 days for the mean patient in the population investigated (n = 19). An estimate of 65.5 L was obtained for the apparent volume of distribution. The elimination half-life of amantadine from brain tissue is much longer than from blood and is comparable to the previously investigated neuroleptic drugs haloperidol and levomepromazine.

NH4+ modulates renal tubule amantadine transport independently of intracellular pH changes

A bicarbonate-dependent organic cation transporter, unique from rOCT1 and rOCT2, primarily mediates amantadine uptake into renal proximal tubules. We examined whether intracellular pH regulates bicarbonate-dependent amantadine transporter function in these tubules. NH(4)Cl treatment resulted in immediate intracellular alkalinization of tubules for up to 30s followed by gradual acidification that was maximal at 5min. Proximal tubule amantadine uptake was similarly inhibited (60%) by NH(4)Cl during both the early intracellular alkalinization and later acidification phases. Sodium propionate treatment resulted in immediate intracellular acidification of proximal tubules without inhibiting amantadine uptake. NH(4)Cl inhibition of bicarbonate-dependent amantadine uptake was dose-dependent, competitive and sex-dependent. NH(4)Cl, NH(4)NO(3), (NH(4))(2)SO(4) and (NH(4))(2)HPO(4) inhibited amantadine uptake into proximal tubules similarly. NH(4)Cl also stimulated efflux of amantadine and tetraethylammonium from preloaded proximal tubules, suggesting mediation of a facilitated process. These data suggest the potential for direct modulation of organic cation transporters by NH(4)(+) in rat kidney proximal tubules.

Single-dose pharmacokinetics of a novel oral platinum cytostatic drug ([OC-6-43]-bis[acetato][1-adamantylamine]amminedichloroplatinum [IV]) in pigs

The pharmacokinetics of total platinum (Pt) were investigated after a single oral dose of (OC-6-43-bis(acetato)(1-adamantylamine)amminedichloroplatinum (IV) (LA-12). A dose of 3 mg/kg (n = 3) and 30 mg/kg (n = 3) was given to two parallel groups of pigs (n = three each). Pt was measured in the blood, urine and feces by atomic absorption spectrometry. Blood was sampled at specified times until 240 h, urine was obtained through a catheter at 1-h intervals until 6 h, and feces were collected until 240 h after administration. LA-12 was rapidly absorbed, as indicated by a T(max) of total Pt within 0.5-1.5 h after administration. The mean (SEM) values for maximum plasma concentration (0.060 +/- 0.025 and 0.39 +/- 0.08 mg/l) and the area under the plasma concentration vs. time curve (12.6 +/- 5.6 and 36.3 +/- 2.0 mg.h/l) increased less than proportionally to the increase in the dose. The mean (SEM) Pt urinary excretion determined over a 6-h postdose period achieved only 1.9% and 0.8% of the administered doses, respectively. Within 2 h after dosing, the renal clearance of total Pt was approximately 2-fold higher than that of creatinine (CL(cr)). Thereafter, it steadily dropped and in the last collection interval (5-6 h postdose) its value was 50% less than CL(cr). Platinum recoveries in feces over 10 days after dosing reached 0.4% and 2.6% of the administered dose, respectively. This finding indicates that the extent of absorption of both doses was high. There were no changes in results of hematology and clinical biochemistry tests.

Simultaneous determination of amantadine and rimantadine by HPLC in rat plasma with pre-column derivatization and fluorescence detection for pharmacokinetic studies

We investigated simultaneous high-performance liquid chromatographic (HPLC) determination of amantadine hydrochloride (AMA) and rimantadine hydrochloride (RIM) levels in rat plasma after fluorescent derivatization with o-phthalaldehyde and 2-mercaptoethanol. Afterwards, the method was applied to determine their pharmacokinetics. The retention times of AMA and RIM derivatives were 12.6 and 22.2 min and the lower limits of detection were 0.025 and 0.016 microg/mL, respectively. The coefficients of variation for intra- and inter-day assay of AMA and RIM were less than 5.1 and 7.6%, respectively. After i.v. administration of AMA or RIM to rats, the total body clearance and distribution volume at the steady-state of RIM were higher than those of AMA. Bioavailability of AMA and RIM was 34.9 and 37.2%, respectively. When AMA and RIM were p.o. co-administered, the area under the plasma concentration--time curve of RIM was significantly lower than that after RIM alone. On the other hand, pharmacokinetic parameters of AMA did not significantly change. These results indicate that our HPLC assay is simple, rapid, sensitive and reproducible for simultaneously determining AMA and RIM concentrations in rat plasma and is applicable to their pharmacokinetic studies. Also, co-administration of AMA and RIM may result in the lack of pharmacological effects of RIM.

Pharmacokinetics and tissue distribution of platinum in rats following single and multiple oral doses of LA-12 [(OC-6-43)-bis(acetato)(1-adamantylamine)amminedichloroplatinum(IV)]

The pharmacokinetics of total and free plasma platinum (Pt) and Pt tissue distribution were investigated in rats after oral administration of (OC-6-43)-bis(acetato)(1-adamantylamine)amminedichloroplatinum(IV) (LA-12). Plasma and ultrafiltrate were sampled until 48 h and tissue samples were taken at 24 and 48 h after single doses of 38.6 or 540 mg LA-12/kg, and after once-a-day dosing of 4.3 or 38.6 mg kg(-1) LA-12 over 14 consecutive days. Total plasma Pt concentrations increased less than proportionally to the 14-fold increase in the single dose. The mean C(max) values of 1.5 and 6.3 mg L(-1) were observed at 0.5 and 1 h, respectively, and the mean AUC values achieved were 29 and 144 mg h L(-1). The highest tissue Pt concentrations were found in the liver and kidneys. Platinum was undetectable in the brain while in other tissues (muscle, skin, heart, lungs), the concentrations were lower (after single dose) or similar (after multiple doses) when compared to the plasma C(max) values. Plasma Pt concentrations after once-a-day dosing of 38.6 mg kg(-1) were two- to three-fold less than that after a single dose while Pt concentrations in various tissues rose two- to four-fold. Accumulation of Pt was even higher in the kidneys (seven-fold) and spleen (nine-fold). After once-a-day dosing, tissue Pt levels increased proportionally with the dose within the range from 4.3 to 38.6 mg kg(-1). At the same time, the increase in total plasma Pt concentrations was 40% less than proportional. Concentrations of Pt in the plasma ultrafiltrate decreased rapidly with the initial half-life of 1 h.

Functional map of TEA transport activity in isolated rabbit renal proximal tubules

The organic cation (OC) transporters OCT1 and OCT2 are suspected of mediating substrate entry from the blood into proximal tubule cells as the first step in renal secretion of OCs. We examined the contribution of each process in different rabbit renal proximal tubule (RPT) segments, taking advantage of the fact that rabbit orthologs of OCT1 and OCT2 can be distinguished by the high affinity of the former for tyramine (TYR) and of the latter for cimetidine (CIM). We verified that TEA uptake, for which both transporters share a similar affinity, is relatively constant in all three segments (apparent inhibitory constant of 33, 74, and 30 microM and maximal rate of mediated TEA uptake of 0.8, 1.0, and 1.2 pmol x mm(-1) x min(-1) in S1, S2, and S3, respectively). In the S1 segment, TYR was a more effective inhibitor of TEA uptake than CIM (IC50 values of 39 and 328 microM, respectively), implicating OCT1 as the predominant pathway for TEA transport. The opposite profiles were noted in the S2 segment (IC50 values of 302 and 20 microM for TYR and CIM, respectively) and S3 segment (IC50 values of 2,900 and 54 microM for TYR and CIM, respectively), suggesting that OCT2 is the predominant TEA transporter in the later portion of RPT. TEA sufficient to saturate OCT1 and OCT2 blocked only 37% of mediated amantadine transport in the S2 segment, confirming the functional presence of at least one additional OC transporter (perhaps OCT3). These data indicate that renal OC transport involves the concerted activity of a suite of transport processes.

Amantadine penetration into cerebrospinal fluid of a child with influenza A encephalitis

The use of amantadine has been advocated as treatment for influenza A encephalitis despite limited information regarding cerebrospinal fluid concentrations and the pathogenesis of encephalitis associated with influenza virus infections. We report a 2-year-old child with influenza A encephalitis treated with amantadine who achieved a potentially therapeutic concentration in cerebrospinal fluid. Despite this the child developed significant neurologic impairment.

Effects of Perioperative Oral Amantadine on Postoperative Pain and Morphine Consumption in Patients after Radical Prostatectomy

Background

Amantadine is known to be a noncompetitive N-methyl-D-aspartate receptor antagonist and may be useful in preventing postoperative central sensitization, acute opioid tolerance, and opioid-induced hyperalgesia, thereby decreasing pain and analgesic requirements. The aim of this pilot study was to evaluate the effects of perioperative oral amantadine on postoperative pain and analgesic consumption.

Methods

Twenty-four patients scheduled to undergo radical prostatectomy were given oral amantadine before and after surgery in a randomized, double-blind, placebo-controlled manner. After surgery, patients received intravenous patient-controlled analgesia with morphine for 48 h. Wound pain intensity, sensitivity to mechanical pressure around the surgical wound, and incidence of bladder spasm pain were assessed. Blood samples were drawn for analysis of amantadine, morphine, and the morphine metabolites. Adverse effects and patient satisfaction were assessed.

Results

The cumulative morphine consumption was significantly lower in the amantadine group at all time points (except at 48 h), amounting to a 32% reduction over the 48-h period. Forty-eight hours after surgery, visual analog pain scores to pressure applied near the wound were significantly lower in the amantadine group than in the placebo group. In addition, the number of patients reporting bladder spasm pain was significantly lower in the amantadine group. Plasma concentration of morphine-3-glucuronide was significantly lower at the end of surgery in the amantadine group. Pharmacokinetic analyses showed that the plasma clearance of morphine at 22-24 h after surgery was also significantly lower in the amantadine group.

Conclusion

The results suggest that perioperative oral amantadine reduces postoperative opioid consumption by pharmacokinetic mechanisms, although additional pharmacodynamic interactions may also be involved.

[Amantadine]

Amantadine is the first antiviral drug for human which was developed by duPont chemical company in 1964. Amantadine causes a selective, dose-related effectiveness of type A influenza virus and cost performance is very high. Amantadine should be given within 48 hours from onset of influenza. Daily doses should not exceed 150 mg in children, 200 mg in adults and less than 100 mg for renal insufficiency. The faulty point of amantadine is that influenza virus becomes easily resistant to the drug through aminoacids change of ionchannel of M2 protein. Fortunately resistant virus is less virulent and has short life in practical meaning. As rapid diagnostic kits are conveniently available for influenza, you should choice amantadine when the patient was positive for type A influenza by kit, especially neuraminidase inhibitor drugs became shortage.

Study of the biodistribution of the amantadine labelled with technetium-99m in Wistar female rats

Amantadine (AMA) has been described as dopamine stimulant and norepineprhine release, capable to block the N-methyl-D aspartate (NMDA) glutamatergic and nicotinic receptors, enhancing the sexual behavior of the male rats and inducing hypersexuality in humans. The use of technetium-99m (99mTc) can be justified for its physical and chemical properties. The aim of this study was to label and evaluate the bioavailability of the AMA labelled with 99mTc (99mTc-AMA) in Wistar female rats. The solution of 99mTc-AMA was administered by intraperitoneal way and the animals were sacrificed in CO2 chamber 10 min after the administration of the radiotracer. Various organs were removed, weighted, their radioactivity was determined using an auto-gamma counter and the results were expressed as the percentage of the injected activity per gram of tissue (%ATI/g). In the control group only Na99mTcO4 was administered. The analysis of results shows that the highest uptakes 99mTc-AMA treated group were: ovary (7.11 +/- 1.43), spleen (3.54 +/- 1.05), thyroid (2.67 +/- 0.15), stomach (1.56 +/- 1.10), duodenum (0.87 +/- 0.52), muscular tissue (0.57 +/- 0.06), liver (0.52 +/- 0.25), and at control group: thyroid (16.45 +/- 2.57), ovary (1.28 +/- 0.12), liver (1.10 +/- 0.04), spleen (0.57 +/- 0.07) and muscular tissue (0.26 +/- 0.03). The results obtained suggest that 99mTc-AMA may be used to study the bioavailability of amantadine and evaluate its effect in sexual behavior in female rats.

Perturbation of rat renal tubule transport of the organic cation amantadine in recent onset streptozotocin-induced diabetes and in uninephrectomy

The effects of early-stage diabetes mellitus and uninephrectomy on the renal tubule transport of amantadine were investigated. Kidney tubules were isolated from streptozotocin-induced diabetic (+/- insulin treatment) uninephrectomized, and control male Sprague-Dawley rats. There were no differences in the Km of amantadine uptake in renal proximal and distal tubules for the imposed treatments compared with control values. Vmax for amantadine uptake in the proximal tubules of diabetic and uninephrectomized rats was higher than the respective control (P < 0.05). Vmax for insulin-treated diabetic rats was similar to control values but was lower than that for untreated diabetic rats (P < 0.05). Vmax for distal tubule uptake was not altered by any treatment. Structure-activity studies demonstrated that bicarbonate-dependent amantadine uptake was inhibited by glycolate and lactate, but not by propionate or alpha-, beta-, or gamma-hydroxybutyrate. Early stage streptozotocin-induced diabetes mellitus and uninephrectomy induced changes in the kidney that resulted in a similar selective increase in proximal tubule amantadine uptake. These data represent the first description that experimentally induced diabetes mellitus and uninephrectomy modulate the function of the renal tubule organic cation (amantadine) transport system. Both interventions represent potential models in which phenotypic modulation of the renal elimination of organic cationic drugs may be achieved and studied.

In vivo analysis of amantadine renal clearance in the uninephrectomized rat: functional significance of in vitro bicarbonate-dependent amantadine renal tubule transport

Amantadine transport into renal proximal and distal tubules is bicarbonate dependent. In the present study, we addressed the effects of bicarbonate on renal clearance and urinary excretion of amantadine. Renal clearance of kynurenic acid was also studied to determine whether bicarbonate effects are specific for organic base transport by the kidney. After a moderate diuresis was established, animals received i.v. [(3)H]amantadine or [(3)H]kynurenic acid followed by an acute dose of sodium bicarbonate or physiological saline. Urine and blood samples were analyzed for [(3)H]amantadine or [(3)H]kynurenic acid, blood gases, and pH. Amantadine and kynurenic acid were excreted by the kidneys, and both compounds underwent renal tubular secretion. Amantadine metabolism occurred, and one metabolite was detected in the urine. In the bicarbonate-treated rats, the total amount of amantadine excreted in the urine was decreased, whereas the amount of metabolite recovered was similar in both groups. Bicarbonate treatment caused a sustained increase in blood bicarbonate levels, a mild increase in blood pH, and a decrease in amantadine renal clearance and in the amantadine/creatinine clearance ratio. Only a transient decrease in the renal clearance of kynurenic acid and the kynurenic acid/creatinine clearance ratio was observed. This study demonstrates that short-term changes in bicarbonate concentration may have significant effects on renal organic cation elimination. Coupled with our previous in vitro demonstration of bicarbonate-dependent organic cation transport, the present study suggests that bicarbonate inhibition of renal tubule organic cation secretion may explain the previous observation that bicarbonate dosing decreases amantadine excretion by the kidney.

Heterogeneity among beta-adrenoreceptor blockers in the modulation of energy-dependent uptake of the organic cation amantadine by rat renal tubules

Eight representative beta-adrenoreceptor blocking drugs, acebutolol, atenolol, labetalol, metoprolol, nadolol, pindolol, propranolol, and timolol, were studied in vitro with respect to their potential to block energy-dependent uptake of [3H]amantadine into proximal and distal rat renal tubule fragments in the presence and absence of bicarbonate. Five of the eight beta-adrenoreceptor blockers showed a dose-dependent inhibition of renal tubule accumulation of amantadine: labetalol, metoprolol, pindolol, propranolol, and timolol. Labetalol was the only beta-adrenoreceptor blocker with greater inhibitory potency in phosphate-based buffer than in bicarbonate-based buffer. Propranolol was the most potent inhibitor of renal tubule amantadine accumulation with IC50 values of 15 +/- 10 and 31 +/- 11 microM for proximal and distal tubule fragments, respectively, in a bicarbonate-based buffer environment. Inhibition in a phosphate-based buffer was less potent only in proximal tubules, with an IC50 of 76 +/- 30 microM. Kinetic studies of propranolol inhibition of amantadine uptake were consistent with both uncompetitive and competitive inhibition mechanisms in bicarbonate-based buffer in both proximal and distal renal tubule segments. There was no chiral preference between the R and S forms of propranolol for the inhibitory effects observed. These data suggest that there is potential for selection among the beta-adrenoreceptor blocking drugs to minimize or restrict the inhibition of amantadine energy-dependent uptake at the organic cation ion uptake sites characterized by amantadine in the presence and absence of bicarbonate.

Brain penetration and in vivo recovery of NMDA receptor antagonists amantadine and memantine: a quantitative microdialysis study

PURPOSE

To determine free brain concentrations of the clinically used uncompetitive NMDA antagonists memantine and amantadine using microdialysis corrected for in vivo recovery in relations to serum, CSF and brain tissue levels and their in vitro potency at NMDA receptors.

METHODS

Microdialysis corrected for in vivo recovery was used to determine brain ECF concentrations after steady-state administration of either memantine or amantadine. Additionally CSF, serum, and brain tissue were analyzed.

RESULTS

Following 7 days of infusion of memantine or amantadine (20 and 100 mg/kg/day respectively) whole brain concentrations were 44-and 16-fold higher than free concentrations in serum respectively. The free brain ECF concentration of memantine (0.83 +/- 0.05 microM) was comparable to free serum and CSF concentrations. In case of amantadine, it was lower. A higher in vivo than in vitro recovery was found for memantine.

CONCLUSIONS

At clinically relevant doses memantine reaches a brain ECF concentration in range of its affinity for the NMDA receptor and close to its free serum concentration. This is not the case for amantadine and different mechanisms of action may be operational.

Possible use of amantadine in depression

Amantadine, originally used in the treatment and prophylaxis of influenza infection, has also proved beneficial in drug-induced Parkinsonism, Parkinson's disease, traumatic head injury, dementia, multiple sclerosis and cocaine withdrawal. Amantadine appears to act through several pharmacological mechanisms, none of which has been identified as the one chief mode of action. It is a dopaminergic, noradrenergic and serotonergic substance, blocks monoaminoxidase A and NMDA receptors, and seems to raise beta-endorphin/beta-lipotropin levels. However, it is still uncertain which of these actions are relevant in therapeutic doses. One new aspect is the antiviral effect of amantadine on Borna disease virus, which it is suspected may possibly play a role in affective disorders. All of these actions could constitute an antidepressant property, and it is suggested that amantadine might work as an antidepressant not through one, but through several mechanisms thought to be related to antidepressant activity. Effects of amantadine on symptoms of affective disorders have been demonstrated in several trials administering it for varying purposes. Additionally, animal studies as well as clinical trials in humans have hinted at an antidepressant activity of amantadine. We present here an overview of the current data. However, only a limited body of evidence is available, and further studies are needed to investigate the efficacy of amantadine as well as its modes of action in depression.

Amantadine and equine influenza: pharmacology, pharmacokinetics and neurological effects in the horse

Amantadine is an antiviral agent effective against influenza A viruses. We investigated 1) the antiviral efficacy, 2) analytical detection, 3) bioavailability and disposition, 4) pharmacokinetic modelling and 5) adverse reactions of amantadine in the horse. In vitro, amantadine and its derivative rimantadine suppressed the replication of recent isolates of equine-2 influenza virus with effective doses (EDs) of less than 30 ng/ml. Rimantadine was more effective than amantadine against most viral isolates; we suggest a minimum plasma concentration of 300 ng/ml of amantadine for therapeutic efficacy. In vivo an i.v. dose of amantadine 15 mg/kg bwt produced mild, transient CNS signs which were no longer apparent after 30 min. Amantadine administered at a dose of 15 mg/kg bwt was established as the maximum safe single i.v. dose. However, if repeated i.v. administration of amantadine is required no more than 10 mg/kg bwt t.i.d. should be used. The maximal safe plasma concentration of amantadine was not evaluated but is probably greater than 2000 ng/ml and possibly greater than 4000 ng/ml. On the other hand, horses with lower seizure thresholds, or those on medications that lower seizure thresholds, may be at increased risk of amantadine-induced seizures, which show few premonitory signs and are rapidly fatal. After i.v. administration of amantadine 10 mg/kg bwt, the disposition kinetics were well fitted by a 2-compartment open model. The estimated peak plasma concentration after this dose was about 4500 ng/ml, the volume of distribution at steady-state (Vdss) was (mean +/- s.d.) 4.9 +/- 1.9 l/kg bwt and the beta phase half-life was 1.83 +/- 0.87 h. Computer projections of plasma amantadine concentrations after i.v. administration of amantadine at a dose of 10 mg/kg bwt t.i.d. at 8 h intervals suggest peak plasma concentrations of 4000-5000 ng/ml and troughs of less than 300 ng/ml will be achieved. Amantadine administered orally at 10 mg/kg bwt and 20 mg/kg bwt showed mean oral bioavailability of about 40-60% and a plasma half life of 3.4 +/- 1.4 h; however, there was substantial inter-animal variation in bioavailability. Projections based on the kinetics observed in individual animals suggest that some animals readily maintain effective plasma concentrations of amantadine after oral administration of 20 mg/kg bwt t.i.d. On the other hand, animals in which amantadine is poorly bioavailable may require up to a 6-fold (120 mg/kg bwt) increase in the oral dose to achieve effective blood concentrations. Withholding food for 15 h did not reduce these inter-animal differences in bioavailability. Our results showed that simple dosing with oral amantadine will not yield effective plasma concentrations in all animals. While i.v. administration yielded more reproducible plasma concentrations, care should be taken to see that the seizure threshold is not exceeded. In acute situations, i.v. administration (5 mg/kg bwt) every 4 h should maintain safe and effective plasma and respiratory tract concentrations of amantadine.

L(+)- and D(-)-lactate modulate rat renal tubular accumulation of amantadine in the presence and absence of bicarbonate

The effect of L(+)-, D(-)- and racemic (DL)-lactate on the energy-dependent renal uptake of the achiral organic cation amantadine was determined with purified proximal and distal cortical tubule fragments isolated from rat kidneys. Kinetic parameters for uptake of amantadine were measured, under constant pH, in bicarbonate buffer (Krebs-Henseleit [KHS]), and in lactate buffers (5 mM) with different proportions of the enantiomers. Km for amantadine uptake increased in all lactate buffers compared with KHS for both proximal and distal tubules. Km for uptake in DL-lactate was similar to that in D(-)-lactate for proximal tubules and to L(+)-lactate in distal tubules, but Km in L(+)-lactate was higher than in D(-)-lactate for both tubules. Maximal transport capacity (Vmax) in DL-lactate and mixtures of enantiomers were similar to KHS but higher than in pure L(+)- and D(-)-lactate. In KHS, lactate inhibited energy-dependent amantadine uptake in a biphasic manner. Graded competitive inhibition of amantadine uptake was observed between 1 and 15 mM lactate for both proximal and distal tubules. This first phase (1-15 mM) inhibited 60% of amantadine uptake. The second phase (15-20 mM lactate) showed a much steeper slope and inhibited the remaining amantadine uptake. There were no differences in inhibitory potencies of the lactate enantiomers for either proximal tubules or distal tubules amantadine tubule uptake. Our present studies suggest that L(+)- and D(-)-lactate modulate amantadine transport by interacting directly with the bicarbonate-dependent transport mechanism(s).

Site-selective effect of bicarbonate on amantadine renal transport: quinine-sensitive in proximal vs quinidine-sensitive sites in distal tubules

The mechanism for bicarbonate enhancement of amantadine (A+) uptake was studied further. A selective modulatory effect of bicarbonate on the stereoselectivity of inhibition of A+ uptake by the stereoisomers, quinine (Q) and quinidine (QD), as reflected by the reversal of potency (Q > QD in KHS vs. QD > Q in phosphate), is reported. Studies were performed in bicarbonate (KHS) and phosphate buffers using purified cortical proximal (PT) and distal tubules (DT) from rat kidneys. Variations of extracellular K+ and Ca++ were used to assess the effect of membrane depolarization and calcium on A+ uptake by the tubules. K+ concentration manipulation (1.5-100 mM) did not change A+ uptake by PT or DT in KHS buffer. High Ca++ (5.0 mM) decreased A+ uptake by PT in KHS (P < .001), whereas in phosphate buffer, both 2.5 and 5.0 mM Ca++ decreased uptake (P < .001). In DT, 0.25 mM Ca++ enhanced A+ uptake (P < .05) in KHS, but 2.5 and 5.0 mM Ca++ decreased it in both buffers. Inhibition studies with Q and QD were performed to characterize the bicarbonate-dependent and bicarbonate-independent A+ transport sites further. Stereoselectivity of inhibition was observed in PT in all buffers used. Potency of Q was higher than QD in KHS but lower than QD in phosphate-buffered PT. QD potency remained unchanged. In phosphate-plus-bicarbonate-buffered PT, the inhibitory potencies for Q and QD increased, with the potency of QD being greater than in KHS (P < .001). For DT, Q and QD were equipotent in all buffers used.(ABSTRACT TRUNCATED AT 250 WORDS)

Rimantadine: a clinical perspective

OBJECTIVE

To provide a review of rimantadine, including its antiviral activity, pharmacokinetics, efficacy, adverse effects, drug interactions, and dosage and administration. Information on influenza A virus and clinical features of influenza disease are presented. Comparative data on rimantadine and amantadine are described.

DATA SOURCES

A MEDLINE search restricted to English-language literature published from 1966 through 1994 and an extensive review of journals was conducted.

DATA EXTRACTION

The data on antiviral activity, pharmacokinetics, adverse effects, and drug interactions were obtained from various articles on rimantadine in open and controlled studies. Controlled double-blind studies were evaluated to assess the efficacy of rimantadine in prophylaxis and treatment of influenza A infection.

DATA SYNTHESIS

Over 90% of a rimantadine dose was absorbed in 3-6 hours in healthy adults. Steady-state plasma concentrations have ranged from 0.10 to 2.60 micrograms/mL at doses of 3 mg/kg/d in infants to 100 mg twice daily in the elderly. Nasal fluid concentrations of rimantadine at steady-state were 1.5 times higher than plasma concentrations, which may explain the effectiveness of rimantadine despite a low plasma concentration. Over 75% of a rimantadine dose was metabolized in the liver, and the parent compound and metabolites were almost completely eliminated by the kidneys. The elimination half-life ranged from 24.8 to 36.5 hours, which allows once-daily dosing. Dosage adjustment is recommended for patients with severe renal impairment (creatinine clearance < or = 0.17 mL/s), severe hepatic dysfunction, or elderly nursing home patients. Drug-resistant strains of influenza A virus to rimantadine occurred in several studies with children and/or adults. Clinical significance of drug-resistant strains has not been established. Rimantadine appeared to be effective in 85-90% of individuals for prevention of influenza A illness and in 50-65% for prevention of influenza A infection. Rimantadine reduced the time to a 50% reduction in symptoms by 1-3 days versus placebo. Differences in symptom reduction between rimantadine and placebo after the first 3 days of treatment was not generally clinically significant. The most common adverse effects of rimantadine administration were associated with the central nervous system (CNS) and the gastrointestinal (GI) tract. CNS-related adverse effects occurred in 3.2% of children younger than 10 years of age and 8.4% of adults. In elderly patients, the incidence of CNS-related adverse effects ranged from 4.9% at 100 mg/d to 12.5% at 200 mg/d. GI adverse effects occurred in 8.4% of children younger than 10 years of age, 3.1% of adults, and 2.9% at 100 mg/d and 17.0% at 200 mg/d in the elderly.

CONCLUSIONS

Rimantadine offers some desirable features for the treatment and prophylaxis of influenza A infection. It appears to be an attractive choice in elderly patients with a history of CNS adverse effects from amantadine and in patients with mild or moderate renal impairment. Although approved for twice-daily dosing, rimantadine has a pharmacokinetic profile that would allow once-daily dosing. It is effective for prophylaxis (not postexposure prophylaxis) and treatment of influenza A virus. It also has a low incidence of adverse effects.

Chronic tobacco smoking and gender as variables affecting amantadine disposition in healthy subjects

Amantadine HCl (3 mg kg-1) was administered orally to 20 young healthy adults. Its apparent volume of distribution (V2/F) was higher in smokers than nonsmokers, 6.05 +/- 0.86 vs 4.87 +/- 0.85 l kg-1; (mean +/- s.d., 10/group, P < 0.011), and no gender-associated effect was observed. Renal clearance did not vary with time-interval, but urinary recovery at 48 h was higher in men than in women (60.2 +/- 7.5% vs 47.0 +/- 15.0%, P < 0.032). Males had higher renal clearances than females when normalised for body mass index (BMI, 0.492 +/- 0.284 vs 0.248 +/- 0.137 l-1 BMI h-1, (10/group, P < 0.032)). On combining data from a previous study, the weight normalised renal clearance was also higher in men than in women, 0.160 +/- 0.075 vs 0.102 +/- 0.053 l kg-1 h-1 (19/group, P < 0.01). Chronic tobacco smoking did not alter the plasma or renal amantadine clearance. We conclude that gender and tobacco smoking are independent variables effecting amantadine disposition.

Amantadine and rimantadine prophylaxis of influenza A in nursing homes. A tolerability perspective

Amantadine and rimantadine are recommended for the treatment and prophylaxis of influenza A infections, and constitute an integral component of influenza control measures in the nursing home setting. However, optimal use necessitates a thorough understanding of the toxicity profiles of these agents, as well as strategies to reduce the risk of adverse reactions. Adverse reactions of these compounds predominantly involve the gastrointestinal tract and the central nervous system (CNS), including hyperexcitability, slurred speech, tremors, insomnia, dizziness, mood disturbance, ataxia, psychosis and fatigue. Based on data from comparative trials, rimantadine appears to exhibit a lesser propensity to cause adverse CNS reactions than amantadine, but a similar propensity to cause adverse gastrointestinal reactions. Factors enhancing the risk of adverse reactions to these agents include reduced renal function (especially for amantadine), drug-drug interactions with cationic drugs, which inhibit amantadine renal tubular secretion (e.g. trimethoprim, triamterene, and possibly cimetidine and procainamide), elevated peak and trough plasma concentrations, and a history of seizures. Careful attention to published dosage adjustment guidelines for these compounds, avoidance of interacting drugs and avoiding these agents in patients with a history of seizures may be the best means to reduce the risk of toxicity in elderly patients. Rimantadine may have an advantage over amantadine in the elderly population in light of its lesser propensity to cause adverse reactions, less complex dosage adjustment in the case of renal impairment and probable lack of drug-drug interaction potential with cationic drugs.

Gender-associated differences in rat renal tubular amantadine transport and absence of stereoselective transport inhibition by quinine and quinidine in distal tubules

The present studies compared male and female rat renal proximal and distal tubular uptake of amantadine, in relation to their transport kinetics and enantioselective inhibition by two diastereoisomers, 8S,9R-(-)-quinine and 8R,9S-(+)-quinidine. Under control conditions, amantadine was concentrated by both tubule fractions with a gender difference for distal tubules (tissue:medium ratio, 18.0 +/- 1.4 for males and 11.0 +/- 0.6 for females; mean +/- S.E.M., P < .05). This was reflected by a higher Km value only in female distal vs. proximal tubular tissue (153 +/- 8 vs. 108 +/- 9 microM; P < .01) but decrease in Vmax values in distal compared to proximal tubules (P < .01) showed no gender-related difference. In proximal tubules, 8S,9R-(-)-quinine and 8R,9S-(+)-quinidine competitively inhibited amantadine transport with apparent inhibitory potency of 2- to 3-fold in favor of 8S,9R-(-)-quinine (P < .01) and without gender preference. Conversely in distal tubules, competitive inhibition of amantadine transport was also elicited by either 8S,9R-(-)-quinine or 8R,9S-(+)-quinidine at a similar concentration range (10-1000 microM), with absence of chiral or gender preference. The present transport data have demonstrated an apparent absence of stereoselectivity in distal tubular uptake inhibition of amantadine, and are suggestive of disparate pathways and/or rate limiting steps involved between renal proximal and distal tubular handling of chiral organic cations for both genders, and between genders for distal tubular transport of amantadine.

Determination of amantadine in human plasma by capillary gas chromatography using electron-capture detection following derivatization with pentafluorobenzoyl chloride

A specific, sensitive, and accurate capillary gas chromatographic method for the quantitation of amantadine in human plasma is described. Amantadine and the internal standard, rimantadine were extracted from plasma under alkaline conditions into toluene. Both compounds were derivatized with pentafluorobenzoyl chloride. The derivatives were separated on a HP-1 capillary column at 180 degrees C and detected using a 63Ni electron-capture detector. The minimum quantifiable limit of the assay is 2.3 ng ml-1 of amantadine base using 1 ml of plasma. The method was used to evaluate the bioequivalence of two different formulations of amantadine hydrochloride.

Gender and age as factors in the inhibition of renal clearance of amantadine by quinine and quinidine

We studied the short-term effect of oral doses of quinine and quinidine on the renal clearance of amantadine in healthy young (age range, 27 to 39 years) and older (age range, 60 to 72 years) adults of both genders in a three-limbed randomized crossover study. Renal clearance of amantadine (13.2 +/- 5.8 L/hr) was significantly inhibited by quinine (9.7 +/- 4.8 L/hr) and quinidine (8.9 +/- 4.0 L/hr) only in male subjects and was not associated with age. The chiral selectivity for the renal clearance of quinidine over quinine was confirmed and extended with the suggestion of both age- and gender-associated changes on the renal clearance ratio for these two diastereomeric drugs. These data support the continued use of amantadine for studies on the renal elimination of organic cationic drugs.

Stereoselective inhibition of renal organic cation transport in human kidney

Amantadine was found to be concentrated by human cortical slices, with apparent Km and Vmax values of 187 +/- 11 microM and 1.37 +/- 0.28 nmol mg-1 min-1 respectively (mean +/- s.e. mean, n = 4). Addition of quinine (8S, 9R-(-)-isomer) or quinidine (8R, 9S-(+)-isomer) competitively inhibited this accumulation (apparent Ki values of 261 +/- 44 (n = 4) and 586 +/- 68 microM (n = 3), respectively). The stereoselectivity of this inhibition is the reverse of that seen with respect to the renal clearances of the diastereoisomers.

[Amantadine and rimantadine against influenza A]

Amantadine and the analogue rimantadine have an antiviral effect on influenza A virus and are approximately 60% effective in preventing illness. The drugs are administered orally, and peak plasma concentration is achieved at two hours after a single dose. Side effects occur in 5-20% of the cases, but generally mild and transient and seen mainly with doses of more than 200 mg a day. This paper describes the mechanism of action and the pharmacokinetics of the drugs, and refers to some important clinical trials. Amantadine has been used in Norway to treat Parkinson's disease since 1972. The licensing of the amantadine and rimantadine for use against influenza A in this country is also discussed.

Antivirals for the chemoprophylaxis and treatment of influenza

Influenza virus infections are one of the leading causes of morbidity and mortality in the United States. Several antiviral agents, amantadine, rimantadine, and ribavirin, have been shown to be either therapeutically or prophylactically effective in influenza virus infections. Amantadine and rimantadine are effective, via the oral route, in treating and preventing influenza A infections. Aerosolized preparations of amantadine and rimantadine have also shown therapeutic efficacy against influenza A. Oral ribavirin has slight therapeutic efficacy in influenza A, but has also shown promising results in therapy of influenza B infections. Aerosolized ribavirin has also shown promise in treatment of patients who are severely ill with influenza A and B.

Effects of chronic amantadine hydrochloride ingestion on its and acetaminophen pharmacokinetics in young adults

The authors studied the effect of chronic amantadine ingestion on its own disposition and that of acetaminophen in five healthy young adults. The half-life of amantadine after 42 days ingestion was 15.1 +/- 2.3 hours and was not different from 14.8 +/- 4.4 hours after an acute ingestion (mean +/- SD). However, chronic amantadine ingestion was associated with an increased apparent volume of distribution for acetaminophen, 1.1 +/- 0.1 L/kg compared with 0.9 +/- 0.1 L/kg, when the two drugs were concurrently ingested after a 2-week washout period. This difference in kinetic distribution was not reflected in terminal acetaminophen half-life, 149 +/- 54 versus 151 +/- 55 minutes for chronic and acute amantadine ingestion, respectively. Plasma acetaminophen clearance with chronic amantadine ingestion (5.8 +/- 2.6 mL/min/kg) was not different from that determined after acute coingestion of both drugs (4.3 +/- 1.1 mL/min/kg). Thus, no change in recommended dose is necessary when these two drugs are coingested.

Differential effects of histamine H2 receptor antagonists on amantadine uptake in the rat renal cortical slice, isolated proximal tubule and distal tubule

The interaction between amantadine and two histamine receptor antagonists was examined in the rat kidney. Amantadine (10 microM, 30 sec) was actively accumulated by cortical slices (slice/medium ratio = 0.4 +/- 0.3 [3.3 +/- 0.3 at 4 min], mean +/- S.E.M.), isolated proximal tubules (tubule/medium ratio = 35 +/- 1) and distal tubules (tubule/medium ratio = 19 +/- 2). In cortical slices, low cimetidine concentrations facilitated amantadine accumulation, whereas higher concentrations produced inhibition. Uptake in proximal tubules was enhanced by cimetidine and reached a maximum at approximately 100 microM. Cimetidine (20 microM) decreased the apparent Km (88 +/- 5 to 55 +/- 3 microM, P less than .005) without altering Vmax (6.8 +/- 0.5 to 5.8 +/- 0.6 nmol/mg/min). Conversely, cimetidine did not enhance uptake in distal tubules but elicited competitive inhibition at concentrations greater than 1 mM. Although this may partially delineate the differences observed between the cortical slice and proximal tubule data, such a discrepancy may also implicate additional sites of interaction in other segments of the cortical nephron and/or cimetidine inhibition of the relatively more significant luminal amantadine efflux in the proximal tubules. Ranitidine did not enhance amantadine accumulation but produced inhibition at high concentrations. In proximal and distal tubule preparations, ranitidine (10 mM) increased Km from 86 +/- 7 to 121 +/- 8 and 95 +/- 5 to 160 +/- 10 microM, respectively (P less than .05), whereas Vmax was not changed (8.9 +/- 0.7 to 7.9 +/- 0.8 and 4.3 +/- 0.1 to 3.8 +/- 0.2 nmol/mg/min, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of amantadine and desipramine combined with psychotherapy for treatment of cocaine dependence

We conducted a single-blind, random assignment, placebo-controlled, 12-week comparison of desipramine hydrochloride and amantadine hydrochloride as adjunctive treatments to counseling for cocaine dependence. Subjects were 54 outpatients who met DSM III-R criteria for active cocaine dependence and who completed a minimum of 2 weeks of treatment. Subjects treated with fixed doses of 200 mg/day desipramine (N = 17), 400 mg/day amantadine-placebo (N = 16), and placebo (N = 21) did not differ for lifetime cocaine use, lifetime histories of psychopathology, admission scores on psychometric assessments, and sociodemographics. All treatment groups demonstrated dramatic and persistent decreases in cocaine use, craving for cocaine, and psychiatric symptoms consequent to treatment. Although there was a trend for more dropouts by subjects taking desipramine, there were no significant differences among treatment groups regarding retention in treatment, craving for cocaine, and decreased cocaine use confirmed by urine toxicology. There was a trend for subjects treated with desipramine to maintain longer periods of cocaine abstinence. Mean plasma concentration of desipramine in a subsample of our subjects was less than that recommended for treatment of depression, thus the dosage of desipramine may have been subtherapeutic.

Pharmacokinetic profile of the immunomodulating compound adamantylamide dipeptide (AdDP), a muramyl dipeptide derivative in mice

A pharmacokinetic profile of 14C-AdDP with uniformly labelled alanine was investigated. It was shown that the distribution phase after an i.v. administration is very short with a half-life of 2.1 min. The half-life of elimination phase after the i.v. administration is about 2.85 hours, that is longer than those of MDP and its derivatives. The total body clearance (30 ml/min/kg) is caused predominantly by metabolism of the compound. All the radioactivity found in urine in a 48 hours interval after a s.c. administration represents only 3.1% of the administered dose. Only a smaller part of the excreted radioactivity is formed by unmetabolised AdDP. The concentration curve after a s.c. administration is characterized by a very fast absorption with a half-life shorter than 1 minute. The distribution and elimination phases are prolonged (20 min, 11 hours respectively) in comparison with an i.v. injection. The decreased absolute bioavailability after a s.c. administration (65%) is probably not biologically significant because of a slower release of the compound from the site of the s.c. administration. A relatively very high radioactivity was found in liver, kidney, thymus, spleen and brain very soon which suggest a very good penetration into tissues. It is an agreement with the high apparent distribution volume of peripheral compartment and higher lipophilicity of AdDP as compared to MDP.

Stereoselective inhibition of amantadine accumulation by quinine and quinidine in rat renal proximal tubules and cortical slices

The renal organic cation transport system was examined. The accumulation of a nonchiral cation, amantadine, by rat renal proximal tubules and cortical slices was investigated, together with the effects of two diastereoisomers, quinine and quinidine. The proximal tubules actively concentrated amantadine with a tissue/medium ratio of 96.3 +/- 1.7 (mean +/- S.E.M., n = 18). Apparent Km was 85 +/- 2 microM and Vmax was 8.0 +/- 0.2 nmol/mg of tubular protein per min. Amantadine accumulation was inhibited competitively by quinine and quinidine with Ki values of 32 +/- 3 and 84 +/- 11 microM, respectively (n = 4). Amantadine was also concentrated by renal cortical slices with tissue/medium ratio of 3.3 +/- 0.3 (n = 4). Apparent Km and Vmax were 94.0 +/- 5.2 microM and 1.27 +/- 0.08 nmol/mg of tubular protein per min, respectively (n = 10). Quinine and quinidine again inhibited amantadine accumulation competitively by the slices, with Ki values of 368 +/- 28 and 780 +/- 84 microM, respectively (n = 4). A similar affinity (Km) for amantadine was observed in both preparations. However, the lower Vmax value in the slice system may be due to additional amantadine transport sites with lower capacity, lesser luminal accumulation and/or limited substrate(s) penetration in the cortical slices. In either preparation, quinine and quinidine functioned as competitive inhibitors and stereoselectivity was observed for the (-)-isomer, quinine, over the (+)-isomer, quinidine. Additional transport sites, reduced luminal substrate accumulation and/or diffusional restraints in the slices are also feasible mechanisms in explaining the differences in Ki values between the two preparations, and their relative contributions await further investigation.

High-performance liquid chromatographic determination of amantadine in urine after micelle-mediated pre-column derivatization with 1-fluoro-2,4-dinitrobenzene

Cationic micelles have been used for the derivatization of the anti-Parkinson drug amantadine with the chromophore 1-fluoro-2,4-dinitrobenzene in urine. In the presence of 90 mM cetyltrimethylammonium bromide (CTAB), the conversion of amantadine into its derivative is complete within 4 min at 60 degrees C and pH 11. Such a short reaction time allows a fully automated pre-column derivatization of amantadine in an on-line combination with reversed-phase high-performance liquid chromatography. This cannot be attained when using purely aqueous derivatization mixtures because then the reaction takes some 20 min at the same temperature. Without the use of an internal standard, the repeatability of the automated determination at the 0.5 microgram ml-1 level is ca. 6%, whilst the detection limit is 75 ng ml-1 (S/N = 3). The present study clearly demonstrates that micellar systems can be beneficially used for the on-line precolumn derivatization of amines in urine.

Amantadine sulphate in treating Parkinson's disease: clinical effects, psychometric tests and serum concentrations

Intravenous administration of amantadine has been shown to be a quick-acting and highly effective method of treating patients with Parkinson's disease. The duration of this therapeutic effect during long-term oral treatment has, however, remained controversial. Therefore, the effect of intravenous treatment was compared with long-term oral treatment over a period of 6 months. Clinical scores and psychometrically determined dexterity improved significantly during 10-day infusion therapy (200 mg amantadine sulphate/day). This improvement was successfully maintained by oral treatment (600 mg/day). A decrease in effectiveness was not observed. Reaction times were within the normal range for the age group involved and did not change significantly during the course of the study. Amantadine serum concentration during the infusion period ranged between 500 and 1000 micrograms/l and produced values nearly half as high as those obtained during oral treatment (600 mg/day). There was a constant relationship (quotient: 1.3) between serum and cerebrospinal fluid concentration. Considerable inter-individual variations were noted. A significant inter-individual correlation between serum concentration and clinical and psychological improvement was not discernible.

[Intravenous and oral treatment with amantadine sulfate in Parkinson disease]

The rapid efficacy of amantadine on akinesia and rigidity in Parkinson's disease is generally known. The duration of this therapeutic result is however controversial. Some authors have reported a loss of efficacy after only a few weeks of therapy. The success of a short-term parenteral and subsequently oral long-term treatment with amantadine sulphate in 8 Parkinsonian patients was tested by means of clinical and neuropsychological examinations and by monitoring the serum concentration over the course of half a year. A ten-day intravenous treatment with amantadine sulphate (200 mg daily) led to a significant improvement in the clinical and psychological test results. This attained improvement could be maintained for 6 months with oral therapy consisting of 600 mg amantadine sulphate. There were strong interindividual variations in serum concentration.

Transport of amantadine and rimantadine through the blood-brain barrier

The unidirectional transport of amantadine and rimantadine across cerebral capillaries, the anatomical locus of the blood-brain barrier, was measured with an in situ rat brain perfusion technique. Both rimantadine and, to a lesser extent, amantadine were transported principally across the blood-brain barrier by a saturable transport system with a one-half saturation concentration of about 1.0 mM (either rimantadine or amantadine). The permeability surface area constants were 8.5 x 10(-2) sec-1 (rimantadine) and 1.1 x 10(-2) sec-1 (amantadine) with concentrations of less than 1.0 microM in the perfusate. The extraction of rimantadine and amantadine from the perfusate at low concentrations (less than 1.0 microM) was 88 and 26%, respectively, of the extraction of diazepam which is 100% extracted. Amantadine and rimantadine transport through the blood-brain barrier was significantly inhibited by weakly basic drugs (e.g., diphenhydramine) but not choline (10 mM), probenecid (1 mM) or leucine (1 mM). Inasmuch as both the pKa and percentage ionized at pH = 7.4 of rimantadine (10.4 and 99.9%, respectively) are much higher than that of amantadine (pKa = 9.0 and 97.5%, respectively), and inasmuch as rimantadine is transported into brain much more readily than amantadine, our results suggest that the carrier-mediated transport of the ionized moiety is the crucial process determining the penetration of amantadine and rimantadine through the blood-brain barrier.

Clinical pharmacokinetics of amantadine hydrochloride

Amantadine is a drug with diverse uses ranging from prevention of influenza A illness to the treatment of patients with Parkinson's disease. It is available only in oral formulations from which it is well absorbed and widely distributed, little drug being present in the circulation. Apparent volume of distribution is inversely related to dose over the therapeutic range and accounts in part for a noteworthy logarithmic increase in plasma concentration as a function of dose. Elimination is primarily by renal clearance by both glomerular filtration and tubular secretion. Amantadine accumulates in patients with renal dysfunction. Hence, doses must be reduced in such patients to avoid toxicity. Interactions with other drugs appear uncommon. Relationships have been demonstrated between amantadine therapeutic effects and plasma concentrations in different study cohorts, but not in individual patients. Dose schedules have been suggested for individuals in whom amantadine kinetics are different from healthy subjects. However, these schedules are controversial in their choice of target concentrations and in being untested as to predictive value.