Pharmacology: Preclinical studies have shown that lithium alters sodium transport in nerve and muscle cells and effects a shift toward intraneuronal metabolism of catecholamines, but the specific biochemical mechanism of lithium action in mania is unknown.
Lithium is inactive in most screening psychopharmacological tests but it produces marked potentiation of amphetamine hyperactivity in animals. It does not appear to protect against the action of stimulant and convulsive drugs and produces only slight potentiation of CNS depressants.
Lithium can replace sodium in extracellular fluid and during the process of depolarization it has an extremely rapid intracellular influx. However, it is not effectively removed by the sodium pump, thereby preventing the cellular re-entry of potassium. As a result, it interferes with electrolyte distribution across the neuronal membrane, leading to a fall in membrane potential and changes in conduction and neuronal excitability. In humans, lithium alters the excitability of the CNS as measured by cortical-evoked potentials.
Balance studies indicate that lithium may produce a transitory diuresis with increase in sodium and potassium excretion. A period of equilibrium or slight retention may follow, but persistent polyuria may occur in some patients.
There is evidence that therapeutic doses of lithium decrease the 24-hour exchangeable sodium. Longitudinal metabolic studies have demonstrated cumulative lithium retention in some patients without undue rise in plasma lithium values, indicating a possible intracellular retention of lithium. There is some evidence that lithium may affect the metabolism of potassium, magnesium and calcium.
There is evidence to indicate that lithium might produce a shift in norepinephrine metabolism from o-methylation to intraneuronal deamination, as evidenced by a decrease in normetanephrine and an increase in deaminated catechols observed in animal studies. This would suggest that lithium may decrease levels of norepinephrine available at the central adrenergic receptors. It would appear, however, that this action is not specific of lithium. Lithium may also alter the metabolism of other monoamines such as serotonin.
EKG changes with lithium have been reported in both animals and man.
Pharmacokinetics: Lithium ions are rapidly absorbed from the gastrointestinal tract and plasma lithium peaks are reached two to four hours after lithium administration. The distribution of lithium in the body approximates that of total body water, but its passage across the blood-brain barrier is slow and at equilibration the CSF lithium level reaches only approximately half the plasma concentration.
Lithium undergoes a biphasic elimination pathway with an alpha half-life of 5 hours and beta half-life of 18 hours.
Lithium is excreted primarily in urine with less than 1% being eliminated with the feces. Lithium is filtered by the glomeruli and 80% of the filtered lithium is reabsorbed in the tubules, probably by the same mechanism responsible for sodium reabsorption. The renal clearance of lithium is proportional to its plasma concentration. About 50% of a single dose of lithium is excreted in 24 hours. A low salt intake resulting in low tubular concentration of sodium will increase lithium reabsorption and might result in retention or intoxication.
Renal lithium clearance tends to be remarkably constant in the same individual but decreases with age and falls when sodium intake is lowered. The dose necessary to maintain a given concentration of serum lithium depends on the ability of the kidney to excrete lithium. However, renal lithium excretion may vary greatly between individuals and lithium dosage must, therefore, be adjusted individually. In clinical reports, it has been noted that serum lithium may rise an average of 0.2 to 0.4 mEq or mmol/L after intake of 300 mg and 0.3 to 0.6 mEq or mmol/L after intake of 600 mg of lithium carbonate. It has been suggested that manic patients retain larger amounts of lithium during the active manic phase, but recent studies have been unable to confirm a clear difference in excretion patterns. However, patients in a manic state seem to have an increased tolerance to lithium.
Comparative Bioavailability Studies: Comparative bioavailability studies were carried out to compare the pharmacokinetic parameters of pms-LITHIUM CARBONATE 300 mg Capsules vs CARBOLITH 300 mg capsules in the fed and fasting states. The results of these studies are presented in Tables 1 and 2. (See Tables 1 and 2.)
Click on icon to see table/diagram/image
Click on icon to see table/diagram/image
The results of these studies demonstrate that pms-LITHIUM CARBONATE 300 mg Capsules are bioequivalent to CARBOLITH 300 mg Capsules.
Once daily administration: Clinical trials comparing once daily at bedtime dosing versus 2-4 times-a-day dosing have shown that urinary volume is significantly decreased with single daily dosing.
Total daily doses of lithium required to reach therapeutic levels were lower with the once-daily dosage schedule than with the divided dosage schedule.
In addition, administration of a single bedtime dose of lithium may result in initial post-absorptive symptoms - which are believed to be associated with rapid rise in serum lithium levels, to occur at night while the patient is sleeping.
In one study, significantly less sclerotic glomeruli, atrophic tubules and interstitial fibrosis were observed in patients on a single daily dosage regimen, as compared to patients on a multiple daily dosage regimen.
Toxicology: The oral ED
50 of lithium carbonate in the rat is 635 mg/kg, and in the mouse 650 mg/kg.
Subacute toxicity studies indicate that lithium accumulates faster and death occurs earlier in rats and dogs fed low sodium diets. Dogs given 20 mg/kg/day of lithium chloride showed no signs of toxicity when fed a normal salt diet, but died in 2-4 weeks when fed a low sodium diet. Similar results occurred in rats. The signs of toxicity consisted of tremors, lethargy, salivation, vomiting, diuresis, bloody diarrhea, anorexia, emaciation and coma. EKG changes similar to those produced by potassium intoxication, were observed. Animals protected by a high sodium intake developed only polyuria. Serum lithium rose gradually in the animals developing signs of toxicity, while serum potassium levels remained fairly constant. In the final stages, serum lithium values rose rapidly as a result of irreversible renal damage in the terminal stages hyperkalemia and azotemia were recorded.
The principal toxic effects of lithium are on the kidney with lesions in the distal convoluted tubule of dogs and in the proximal convoluted tubules of rats. The primary toxic effects in man appear to be on the central nervous system.
The long-term toxicity of lithium has not yet been tested in animal studies.
Reproductive Studies: Lithium salts influenced the embryonal development of sea urchins, mollusks, amphibians, and chicken embryos.
Adverse effects on reproduction have also been reported in mammalian species. Adverse effects on the number of corpora lutea, percentage of resorption, embryonal viability and weaning weights in rats, the number of implantation sites in rabbits, and the birth weights in monkeys, have been produced in lithium studies. Cleft palates occurred in the offspring of treated mice and rats, in the latter species together with ocular and auricular defects, with lithium doses producing blood levels similar to those obtained with therapeutic doses in man.
Lithium decreases the fertility of male rats and is spermicidal in vitro for human and animal spermatozoa.
The retrospective studies congenital abnormalities were observed in 6% of infants born to mothers taking lithium carbonate during the first trimester of pregnancy. This incidence was considered to be no greater than that observed in the general population of infants.
Infants born to mothers who took lithium during pregnancy had a higher than expected ratio of cardiovascular anomalies (6%).