A study of the transport of lithium across the erythrocyte membrane in vivo and of the effects of the ion transport inhibitors digoxin and dipyridamole

Population pharmacokinetic modeling of sustained release lithium in the serum, erythrocytes and urine of patients with bipolar disorder

PURPOSE

Lithium (Li), the first-line treatment of bipolar disorder, was first developed as an immediate-release form with a routine therapeutic drug monitoring 12 h after the last dose. In Europe, the most commonly prescribed form is a sustained release (srLi). Yet no pharmacokinetics (PK) study has been published of srLi, administered once a day, in adults. The present study describes srLi PK in the serum and erythrocytes of bipolar patients.

METHODS

To assess srLi PK, we studied prospectively 17 French bipolar patients on a median dose of 1000 mg (600-1600) for at least 2 years. Serum (S), erythrocyte (E) concentrations, and urinary (U) amount were collected over 8 h after 15 days of morning intake using monitoring electronic medical system (MEMs). Population PK parameters were estimated using the SAEM algorithm (MONOLIX 4.3.3 software).

RESULTS

Using a population approach, we built a PK population model of srLi including one S compartment (V_S = 23.0 L, Cl_S = 1.21 L h^-1), one E compartment (V_E = 64.7 L, Cl_SE = 3.63 L h^-1, Cl_ES = 9.46 L h^-1), and one U compartment (F = 0.62) and estimate the ratio of concentrations to Li in E over S at 0.38 with 27% between-subject variability.

CONCLUSION

This is a PK model of srLi once a day in bipolar patients using a population approach simultaneously describing Li concentrations in serum, erythrocytes, and urine which provide an estimate of the ratio of concentration in erythrocyte over serum and its between-subject variability (BSV).

An attempt to predict daily erythrocyte lithium fluctuations

Erythrocyte lithium concentration, which is a better predictor of brain lithium levels than plasma lithium concentrations, possesses the disadvantage of precise hourly determination following the last intake. The variability in RBC lithium accumulation increases as the extracellular lithium concentration increases. This increase is also time dependent and it would be very useful if the pharmacokinetic rate constant were known. Unfortunately, low lithium levels do not allow measurements within confidence intervals. In this work, we tried to determine, in vitro, the kinetic rate constants in erythrocytes of healthy volunteers. Different high lithium loaded plasma-like media were used for an extrapolation procedure of constants allowing the determination of an erythrocyte load constant namely K0 = 0.0161 +/- 0.0005 h-1 at corresponding plasma lithium concentrations. The abnormalities of lithium transport determined by in vitro procedures would be very useful in understanding the etiology of affective illness. Lithium flux pre-controls corrected with this rate constant would be very helpful in enlarging laboratory time management.

Brain lithium concentrations measured with lithium-7 magnetic resonance spectroscopy in patients with affective disorders: relationship to erythrocyte and serum concentrations

Brain lithium concentrations were measured in eight patients with affective disorders using lithium-7 magnetic resonance spectroscopy (MRS). Brain lithium concentrations correlated better with serum concentrations (n = 23, r = 0.66, p < 0.001) than with erythrocyte concentrations (r = 0.44, p < 0.05). Because of previous data in animal experiments these results were unexpected, but the differences in cation transport mechanisms between neurons and erythrocytes may account for the results.