The plasma disposition and renal elimination of digoxin-specific Fab fragments and digoxin in the rabbit.

Administering digoxin-specific antibody fragments (DSFab, 1.9 mg kg-1, i.v.) to rabbits 1 h after digoxin (15 micrograms kg-1 or 12.5 microCi kg-1, i.v.) produced a redistribution of digoxin associated with a 5-fold elevation in total plasma concentration and 36-86% reductions in elimination half-life, apparent volume of distribution at steady-state and total body clearance (CLT). Renal clearance (CLR) was also reduced (54%), but urinary digoxin excretion was increased by one-third (35% vs 25%). This apparent anomaly is due to the large rise in total plasma digoxin concentration with a consequent increase in the area under the plasma concentration curve (AUC). The AUC, which is the denominator term in calculating CLR (and CLT), was increased to a greater extent than urinary digoxin excretion (numerator term in calculating CLR) so that an overall reduction in CLR occurred. The initial presence of digoxin appeared to alter the distribution of DSFab, since their plasma concentrations were markedly higher when the antibody was given after the hapten. The digoxin also reduced (from 3 to 1%) the amount of detectable DSFab in the urine.

Digoxin-specific Fab fragments impair renal function in the rabbit.

A 2 mg kg-1 intravenous bolus dose of digoxin-specific Fab fragments produced a 28% reduction in creatinine clearance in rabbits after 24 h. Urine output was reduced, while plasma and urinary creatinine concentrations were unaffected and increased, respectively. By 5 days the creatinine clearance had returned to normal. The fractional excretion of Na+ was nearly halved, indicating that the tubular reabsorption of Na+ increased to compensate for the reduced glomerular filtration rate, suggesting that tubular (as opposed to glomerular) function was not impaired.

The plasma kinetics of digoxin-specific Fab fragments and digoxin in the rabbit.

The plasma kinetics of total and free digoxin, and digoxin-specific antibody fragments (DSFab) in rabbits which had been given [3H]digoxin one hour before DSFab has been studied over a 5 day period. Injection of DSFab caused a 4- to 5-fold rise in total digoxin and reduced elimination half-life (t1/2 beta), apparent volume of distribution at steady-state (Vdss) and systemic clearance (CL) by 40, 90 and 75% respectively. Early in the experimental period, DSFab reduced free digoxin concentration (measured by ultrafiltration) from 4.1 ng mL-1 to a minimum of 1.3 ng mL-1 at 15 min. However, the concentration had rebound to 2.5 ng mL-1 by 60 min. Subsequently, free digoxin fell to 0.63 ng mL-1 and remained relatively constant over a 7 to 90 h period. The distribution half-life, t1/2 beta, Vdss and CL for DSFab (concentrations measured by enzyme-linked immunosorbent assay) were 0.3 h, 3.2 h, 185 mL kg-1 and 57 mL kg-1 h-1, respectively. A considerable molar excess (about 5) of DSFab in the plasma was necessary to maintain minimum free digoxin concentrations. When the DSFab:digoxin molar ratio was less than 4 during the initial treatment period, free (toxicologically active) concentrations increased. With the elevation in total digoxin, however, an opposite situation appeared to apply. By 24 h the relatively short DSFab t1/2 beta meant that the plasma DSFab concentration was less than 0.05 micrograms mL-1 giving a DSFab:digoxin molar ratio of below 0.06, yet the antibody-induced rise in total digoxin concentration was still detectable at 100 h.

The use of enzyme-linked immunosorbent assays to study the plasma disposition of sheep polyclonal and rat monoclonal digoxin-specific Fab fragments in the rabbit.

The plasma disposition of sheep polyclonal digoxin-specific Fab (fragment antigen-binding) fragments has been studied in rabbits after their intravenous injection (1 mg kg-1) using enzyme-linked immunosorbent assays exploiting both the species-specificity (ELISA1) and the digoxin-specificity (ELISA2) of digoxin-specific Fab fragments. The log concentration versus time profiles were best described by a biexponential plasma disposition when either assay was used. Although the plasma concentrations determined by ELISA1 and ELISA2 at each sampling time were not significantly different, there was a tendency for certain ELISA2 values to be higher. This resulted in the ELISA2-derived data giving a significantly longer distribution half-life (t1/2 alpha), but similar values for elimination half-life (t1/2 beta), apparent volume of distribution at steady state (Vdss), and clearance. Using ELISA2, which was generally the more sensitive assay, to compare the plasma disposition of the sheep polyclonal digoxin-specific Fab fragments with rat monoclonal digoxin-specific Fab fragments, it was shown that the rat product had a shorter t1/2 alpha (11 vs 22 min), a t1/2 beta which was not significantly different (253 vs 168 min), but a faster clearance (1.2 vs 0.7 mL kg-1 min-1), associated with a much larger Vdss (321 vs 108 mL kg-1). The extracellular fluid volume, using thiocyanate as a marker, was about 216 mL kg-1 for the nine rabbits used. This suggests that the rat preparation penetrates more extensively into the extracellular space and may indicate that some degree of extracellular binding or cell penetration is occurring.

The plasma disposition of sheep antibody (Fab) fragments in the guinea-pig and rabbit.

The plasma elimination of sheep digoxin-specific Fab (fragment antigen-binding) antibody fragments has been studied after intravenous injection (1 mg kg-1) in guinea-pigs and rabbits using an enzyme-linked immunosorbent assay. The log concentration versus time profiles were best described by biexponential and triexponential functions for the response in the guinea-pig and rabbit, respectively. However, the elimination half-lives and apparent volumes of distribution were similar in both species (about 140 min and 120 mL kg-1, respectively). The value for the Fab distribution volume suggests that the antibody fragments distribute out of the vascular compartment but do not fully occupy the extracellular space. Our estimates of the latter, using thiocyanate as a marker, ranged from 220 to 327 mL kg-1 (rabbits and guinea-pigs, respectively). The distribution of Fab fragments in these two species differs significantly from that in the rat, where our earlier studies have shown that these antibody fragments are confined to the intravascular compartment with a distribution volume approximately equivalent to that of plasma (about 40 mL kg-1).