Influence of plasma protein binding on the brain uptake of an antifungal agent, terbinafine, in rats

Free Drug Theory - No Longer Just a Hypothesis?

The Free Drug Hypothesis is a well-established concept within the scientific lexicon pervading many areas of Drug Discovery and Development, and yet it is poorly defined by virtue of many variations appearing in the literature. Clearly, unbound drug is in dynamic equilibrium with respect to absorption, distribution, metabolism, elimination, and indeed, interaction with the desired pharmacological target. Binding interactions be they specific (e.g. high affinity) or nonspecific (e.g. lower affinity/higher capacity) are governed by the same fundamental physicochemical tenets including Hill-Langmuir Isotherms, the Law of Mass Action and Drug Receptor Theory. With this in mind, it is time to recognise a more coherent version and consider it the Free Drug Theory and a hypothesis no longer. Today, we have the experimental and modelling capabilities, pharmacological knowledge, and an improved understanding of unbound drug distribution (e.g. Kp_uu) to raise the bar on our understanding and analysis of experimental data. The burden of proof should be to rule out mechanistic possibilities and/or experimental error before jumping to the conclusion that any observations contradict these fundamentals.

The Effects of Competitive Displacement on Haloperidol's Plasma Distribution in Normolipidemic and Hyperlipidemic Plasma

PURPOSE

To assess whether dyslipidemia affects haloperidol's overall plasma distribution when it is in the presence of another highly protein bound drug that competes for plasma protein binding sites.

METHODS

We performed in vitro studies in which warfarin sodium was pre-incubated in normolipidemic and hyperlipidemic plasma samples in varying concentrations. Following the pre-incubation with warfarin, [3H]-haloperidol mixed with unlabeled haloperidol was added to the plasma samples. The plasma was separated into its lipoprotein and lipoprotein deficient fractions by density gradient ultracentrifugation and haloperidol distribution was determined.

RESULTS

Our results indicate that when normolipidemic plasma was pre-incubated with various concentrations of warfarin no significant redistribution of haloperidol was noted among the various plasma lipoprotein fractions. However, in the case of the hyperlipidemic plasma, pre-incubating with warfarin did result in a significant redistribution of haloperidol from the lipoprotein-deficient fraction to the very-low-density and low-density fractions of lipoproteins.

CONCLUSION

Understanding how plasma lipoproteins influence competitive displacement interactions would be valuable in helping to explain and perhaps predict pharmacokinetic parameters that may affect clinical outcome. The clinical significance of competitive displacement of drugs in patients with dyslipidemia requires further study.

Effects of serum on in vitro susceptibility testing of echinocandins

The effects of protein binding on the activities of caspofungin, anidulafungin, and micafungin were evaluated against Candida and Aspergillus species. Adding human serum sharply increased the MICs of micafungin and anidulafungin and modestly affected the MIC of caspofungin. The increase in MICs does not appear consistent with the rate of protein binding for the three compounds.

Human pharmacogenomic variations and their implications for antifungal efficacy

Pharmacogenomics is defined as the study of the impacts of heritable traits on pharmacology and toxicology. Candidate genes with potential pharmacogenomic importance include drug transporters involved in absorption and excretion, phase I enzymes (e.g., cytochrome P450-dependent mixed-function oxidases) and phase II enzymes (e.g., glucuronosyltransferases) contributing to metabolism, and those molecules (e.g., albumin, A1-acid glycoprotein, and lipoproteins) involved in the distribution of antifungal compounds. By using the tools of population genetics to define interindividual differences in drug absorption, distribution, metabolism, and excretion, pharmacogenomic models for genetic variations in antifungal pharmacokinetics can be derived. Pharmacogenomic factors may become especially important in the treatment of immunocompromised patients or those with persistent or refractory mycoses that cannot be explained by elevated MICs and where rational dosage optimization of the antifungal agent may be particularly critical. Pharmacogenomics has the potential to shift the paradigm of therapy and to improve the selection of antifungal compounds and adjustment of dosage based upon individual variations in drug absorption, metabolism, and excretion.

In-vitro distribution of terbinafine in rat and human blood

The association of drugs with plasma lipoproteins has the potential to influence drug action and disposition. In this study, the uptake and distribution of the lipophilic antifungal drug, terbinafine, was investigated in rat and human blood and plasma. Fresh plasma was incubated with terbinafine (200-1000 ng mL(-1)), then subjected to vertical spin density gradient ultracentrifugation to separate protein fractions. The concentrations of terbinafine in each fraction was determined using a validated reversed-phase HPLC method. The association of terbinafine with very-low-density lipoproteins (15.5 +/- 7.1% of total concentration) in human plasma was significantly lower than that associated with fractions containing soluble proteins (28.0 +/- 6.2%), high- (26.8 +/- 7.7%) and low-density lipoproteins (31.6 +/- 4.6%). In rats terbinafine was found to be distributed evenly through plasma protein fractions. The association of terbinafine in lipoproteins was independent of concentration (over the range 200-1,000 ng mL(-1)) and species. The distribution of terbinafine was examined in human and rat blood and the blood-to-plasma ratio of terbinafine was 0.70+0.09 and 1.01 +/- 0.20, respectively, indicating higher association of terbinafine with plasma components than erythrocytes in humans. This study suggests that in humans and rats, terbinafine associates with a number of plasma proteins independently of terbinafine concentration. Alteration in plasma lipoprotein concentrations are therefore likely to influence terbinafine binding in blood and distribution in the body.

Comparative efficacies of terbinafine and fluconazole in treatment of experimental coccidioidal meningitis in a rabbit model

A rabbit model of coccidioidal meningitis was used to compare the therapeutic efficacies of terbinafine (TBF) and fluconazole (FCZ). Hydrocortisone acetate-treated New Zealand White male rabbits were infected intracisternally with either 2.2 x 10(4) or 6.4 x 10(4) Coccidioides immitis arthroconidia. Oral treatment with polyethylene glycol 200 (PEG) twice daily (n = 8), TBF twice daily (n = 9; 200 mg/kg of body weight/day), or FCZ once daily (n = 8; 80 mg/kg/day) began on day 5 and continued for 21 days. Mean survival times were 20, 24, and 32 days for rabbits treated with PEG, TBF, and FCZ, respectively. All of the FCZ-treated animals (100%; P = 0.003), 56% of the TBF-treated animals (P = 0.4), and 25% of the PEG-treated animals survived the length of the study. Both FCZ and TBF were effective at reducing the incidence of paresis. Only FCZ was effective at reducing most neurological and systemic signs. FCZ treatments resulted in lower cerebrospinal fluid (CSF) protein concentrations and leukocyte counts and faster clearing of CSF fungal cultures compared with those for PEG-treated controls, but TBF treatments had no significant effect on these parameters. Neither drug affected CSF glucose levels. Mean serum TBF levels by bioassay were within the range of 3.5 to 6.2 microgram/ml at 1, 2, and 4 h postdosing and 0.35 to 7.0 microgram/ml at 14 h postdosing. No TBF was detected in CSF. Mean FCZ levels (24 to 25.5 h postdosing) by bioassay were 16.4 to 19.2 and 13.5 to 19.2 microgram/ml in serum and CSF, respectively. The reduction in the numbers of CFU in the spinal cord and brain was over 100-fold (P = 0.0005) in FCZ-treated animals and 2-fold (P </= 0.2) in TBF-treated animals compared with those in PEG-treated animals. Histopathologic severity (semiquantitative scoring system) was significantly attenuated by FCZ treatment (P = 0. 05) and was slightly attenuated by TBF treatment compared with that for the controls. In conclusion, TBF appeared to have a slight effect on survival, histology, and reduction of the numbers of CFU in tissue; however, these effects were not significant. FCZ was effective at controlling coccidioidal meningitis.

Estimation of the binding affinity constants of soluble ligand-receptor complexes by a rapid filtration technique: [3H]-flunitrazepam-bovine serum albumin as an example

A method for determining the equilibrium dissociation constant (KA) of a soluble ligand (L) from a soluble receptor (A) in the presence of another solid phase receptor (R) for the same ligand was developed. The total and nonspecific binding of L to R was measured in the presence and in the absence of A. The separation of bound and free L was done by a rapid filtration technique so that only the complex RL, but not AL, was recovered. An apparent dissociation constant (KR,app) was calculated from the saturation curve obtained in the presence of A. The magnitude of KA could be determined from this KR,app and the value of the equilibrium dissociation constant of the complex R-L (KR) calculated from the saturation curve in the absence of A. The equality of the Bmax values obtained in the presence and in the absence of A assured the accuracy in the determination of KA so that the fulfillment of this condition could be used as an internal control. For the correct definition of nonspecific binding, the displacement agent (L1) should be used at concentrations within the range 10(2).KR < L1 < 10. K4. This fact constraints the applicability of the method to systems where KA/KR > 10(3). The highest sensitivity of the method can be attained when 0.33 < [At]/KA < 3. The equilibrium binding constant of [3H]-flunitrazepam to non-delipidized bovine serum albumin determined by the present approach (31 +/- 7 mumol/L) did not differ significantly from the literature.

Influence of serum protein binding on the in vitro activity of anti-fungal agents

Historically it has been assumed that the pharmacological effect is related to the free drug concentration. In exposing Candida albicans to itraconazole and ketoconazole serum concentration-time profiles, however, antifungal activity was not diminished despite intense albumin binding. The relevance of serum protein binding was further investigated, by in vitro susceptibility testing of C. albicans (40 clinical isolates) and Trichophyton rubrum (ten strains) against antifungal agents using microdilution tests allowing the determination of IC30- and MIC-values. The range of serum protein binding ranges from 11% with fluconazole to > 99% with itraconazole and terbinafine. The ratios of IC30- and MIC-values with and without serum protein (albumin, alpha- and gamma-globulin, human plasma) were related to the loss of susceptibility expected according to the free-drug hypothesis. A difference in the albumin effect with the test strains was not observed. With most antifungals including terbinafine, the activity declined as expected. IC30- and MIC-ratios for miconazole were 7 and 13 (observed) vs. 12-20 (expected), for fluconazole 1.5 and 3.5 vs. 1.1, for amphotericin B 10 vs. 11-20, for griseofulvin 3.6 vs. 4, and for terbinafine 61 vs. 100. Itraconazole activity, however, was not diminished by albumin (expected ratio 286), and ketoconazole effects decreased less than expected (ratio 5-15, expected about 100). alpha-globulin, but not gamma-globulin induced a major loss in anti-Candida activity of itraconazole and ketoconazole, which is paralleled by a decline in ketoconazole (but not itraconazole) activity due to plasma. With the other antifungals (except for ciclopiroxolamine) IC30-values for C. albicans increased, too. Due to the complete inhibition of T. rubrum growth by gamma-globulin, this species proved unsuitable for studying the gamma-globulin effects. The present study demonstrates that the effects of intense protein binding on drug activity are only partly predictable from binding studies in vitro.

Plasma protein binding of the experimental antitumour agent acridine-4-carboxamide in man, dog, rat and rabbit

The plasma binding of N-[2-(dimethylamino)ethyl]acridine-4-carboxamide (AC) was investigated in-vitro by equilibrium dialysis for 3 h at 37 degrees C against isotonic phosphate buffer (pH 7.35) using [3H]AC. There were significant species differences with the smallest % free fraction (mean +/- s.d.) occurring in human plasma (3.4 +/- 0.2), followed by dog (8.1 +/- 0.4), mouse (14.8 +/- 0.8), rat (16.3 +/- 0.9) and rabbit (20.2 +/- 0.7). In plasma from healthy individuals (n = 5), the % free fraction ranged from 2.7 to 3.8. In physiological solutions of human proteins, the greatest binding was observed for alpha 1-acid glycoprotein (AAG) (0.75 g L-1) with a mean free fraction of 24.1 +/- 2.2%, followed by albumin (40 g L-1) with 31.6 +/- 0.7 and 39.8 +/- 2.5% for fatty-acid-free and globulin-free, respectively. There was also some binding to globulins (5 g L-1) with a mean % free fraction of 70.3 +/- 1.6 and 84.8 +/- 2.2 for Cohn's fraction I and IV, respectively. Binding data from the displacement of [3H]AC by increasing concentrations of AC in human AAG (0.75 g L-1) or albumin solution (40 g L-1) indicated that AAG had 10-fold greater binding affinity for AC (Ka, 7.8 x 10(4) M-1) compared with albumin (Ka, 6.8 x 10(3) M-1). In human plasma enriched with AAG there was a significant negative linear correlation (r = 0.932; P < 0.001) between % AC free fraction and increasing AAG concentration over the range 0.6-4.5 g L-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetic optimisation of oral antifungal therapy

The range of oral antifungal therapy has been expanded recently by the introduction of itraconazole, and terbinafine. These agents have a broader spectrum of activity than griseofulvin and flucytosine, and induce less liver toxicity than ketoconazole. Treatment with these agents may be optimised by application of pharmacokinetic principles. Griseofulvin, ketoconazole and itraconazole should be administered with food to ensure adequate absorption. Maximal absorption of griseofulvin is achieved by administration of the drug as a solid solution in polyethylene glycol. Absorption of azole antifungal agents is impaired by high gastric pH, which is observed in some patients with acquired immunodeficiency syndrome. It is also impaired by frequent vomiting, which commonly occurs in patients with neutropenia. Furthermore, antacids, H2-antagonists and sucralfate interfere with absorption of ketoconazole. The newer oral antifungals are more slowly eliminated and associated with less pronounced drug interactions than ketoconazole. As with ketoconazole, itraconazole and fluconazole influence cyclosporin metabolism. These effects are of clinical relevance and necessitate cyclosporin dosage reduction. However, the cyclosporin dosage reduction required during coadministration of itraconazole and fluconazole (50 to 55%) is less than that required when ketoconazole is concomitantly administered (85%). Monitoring of cyclosporin concentrations during coadministration with these agents is necessary to avoid nephrotoxicity. Drug monitoring is also advisable when phenytoin, carbamazepine or rifampicin (rifampin) are administered concomitantly with azoles, due to a mutual influence on drug metabolism. The antifungal activity of itraconazole is not related exclusively to free drug concentrations. Therefore, the low protein binding of fluconazole does not place this agent at an advantage over itraconazole in the treatment of fungal meningitis. However, terbinafine may be superior to itraconazole for the treatment of tinea unguium, another recalcitrant fungal disease, because terbinafine more rapidly penetrates the nail plate. During repeated use, itraconazole concentrations increase slowly in the nail plate. Steady-state concentrations are reached in the stratum corneum only after several weeks' administration. Following cessation of treatment, terbinafine, itraconazole and ketoconazole concentrations in keratinised tissues decline slowly. This allows a short duration of drug treatment. Some clinical trials suggest that low concentrations of flucytosine, griseofulvin and itraconazole are associated with treatment failure. Flucytosine-induced myelotoxicity also appears to be concentration dependent. This adverse reaction may be caused by fluorouracil (which is produced by metabolism of flucytosine by enterobacillary flora in the gut) rather than by the parent compound.

Interaction of terbinafine with human serum and serum proteins

The allylamine antimycotic terbinafine acts by inhibition of ergosterol biosynthesis at the level of squalene epoxidase. Using this mechanism in Candida parapsilosis cells, a functional assay was developed to investigate the effects of serum and serum proteins on the antifungal action of terbinafine and related drugs in vitro. Inhibition of ergosterol biosynthesis by terbinafine was antagonized by human serum in a dose-dependent non-saturable manner. The results were not affected by varying the period of pre-incubation of serum with the drug or with the fungal cells, or by performing the test in other species of Candida, Aspergillus and Trichophyton. Qualitatively similar effects were observed with the related allylamine compounds naftifine and SDZ 87-469, the extent of antagonism correlating with their lipophilicity. The effect appeared to be caused by non-specific binding of the drug to major serum components, including albumin and the lipoproteins (both LDL and HDL). Reduced bioavailability resulting from binding by serum may at least partly account for the low efficacy of terbinafine in experimental models of systemic infection, in contrast to its high efficacy in infections of the skin, nails and hair.