Anatomical-physiological approaches in pharmacokinetics and pharmacodynamics

Binding of new aminopropan-2-ol compounds to bovine serum albumin, α1-acid glycoprotein and rat serum using equilibrium dialysis and LC/MS/MS

BACKGROUND

The binding of three new aminopropan-2-ol compounds briefly called 2F109, ANBL and TWo8 with potential cardiovascular activity to bovine serum albumin (BSA), α1-acid glycoprotein (AGP) and to rat serum was studied. The chemical structures of these compounds are related to carvedilol. They possess an antiarrhythmic and hypotensive activity, and β- and α-adrenolytic mechanism of action. All analogues are weak bases with pKa values 8.65, 8.85 and 8.26 for 2F109, ANBL and TWo8, respectively, and they possess lipophilic character (log P > 1.9584).

METHODS

The extent of protein binding was determined using equilibrium dialysis in the range 2.5 - 900 μM, and 2.5 - 300 μM for binding of investigated compounds to BSA and AGP, respectively, and the quantitative measurement was done by LC/ESI-MS/MS assay.

RESULTS

The studied compounds bound to a single class of binding sites on BSA which was characterized by low affinity (Kd for 2F109 = 8.49 x 10(-5) M, for ANBL= 1.92 x 10(-5) M, and for TWo8 = 1.71 x 10(-5) M) and low capacity (n = 0.53 for 2F109, 0.132 for ANBL and 0.13 for TWo8). The binding of 2F109, ANBL and TWo8 to AGP revealed one class of binding sites, with moderate affinity (Kd for 2F109 = 4.67 x 10(-6) M, for ANBL = 3.48 x 10(-5) M, and for TWo8 = 1.13 x 10(-5) M) and higher capacity (n = 2.21 for 2F109, 2.76 for ANBL and 2.28 for TWo8).

CONCLUSION

The obtained data indicate that 2F109, ANBL and TWo8 moderately bind to BSA (34.2 - 71.2%) with low capacity (Ka = 6.21 x 10(3) - 7.61 x 10(3)M(-1)) and strongly bind to AGP (71.5 - 85.5%) with moderate affinity (Ka = 7.94 x 10(4) - 4.73 x 10(5)M(-1)).

Impact of peripheral elimination on the concentration-effect relationship of remifentanil in anaesthetized dogs

BACKGROUND

This study elucidates the impact of sampling site when estimating pharmacokinetic-pharmacodynamic (PK-PD) parameters of drugs such as remifentanil that undergo tissue extraction in the biophase. The interrelationship between the concentrations of remifentanil predicted for the effect compartment and those measured in arterial, venous, and cerebrospinal fluid were investigated under steady-state conditions.

METHODS

Following induction of anaesthesia with pentobarbital, an arterial cannula (femoral) and two venous catheters (jugular and femoral) were inserted. Electrodes were placed for EEG recording of theta wave activity. Each dog received two consecutive 5-min infusions for the PK-PD study and a bolus followed by a 60-min infusion was started for the steady-state study. Cerebrospinal fluid, arterial and venous blood samples were drawn simultaneously after 30, 40, and 50 min. At the end of the infusion, arterial blood samples were collected for pharmacokinetic analysis.

RESULTS

Remifentanil PK-PD parameters based on theta wave activity were as follows: apparent volume of distribution at steady-state (V(ss)) (231+/-37 ml kg(-1)), total body clearance (Cl) (63+/-16 ml min(-1) kg(-1)), terminal elimination half-life (t(1/2 beta)) (7.71 min), effect compartment concentration at 50% of maximal observed effect (EC(50)) (21+/-13 ng ml(-1)), and equilibration rate constant between plasma and effect compartment (k(e0)) (0.48+/-0.24 min). The mean steady-state cerebrospinal fluid concentration of 236 ng ml(-1) represented 52 and 74% of that in arterial and venous blood, respectively.

CONCLUSIONS

Our study re-emphasizes the importance of a sampling site when performing PK-PD modelling for drugs undergoing elimination from the effect compartment. For a drug undergoing tissue elimination such as remifentanil, venous rather than arterial concentrations will reflect more exactly the effect compartment concentrations, under steady-state conditions.

Pharmacokinetics of Local Anaesthetics in Infants and Children

Amide local anaesthetics used for regional anaesthesia in paediatric patients are potent sodium channel blockers with marked stereospecificity, which consistently influences their action, especially their toxic action on the heart. At toxic concentrations, they induce severe arrhythmias with the potential for cardiac arrest. These agents are all bound to serum proteins, mainly to alpha(1)-acid glycoprotein (AAG), but also to human serum albumin. Protein binding ranges from 65% (lidocaine) to more than 95% (bupivacaine, ropivacaine). Because AAG is a major acute phase protein, its concentration rapidly increases when inflammatory processes develop, particularly during the postoperative period. Neonates and infants have a lower AAG concentration in serum as compared with adults; therefore, their free fraction of local anaesthetics is increased accordingly. This has important clinical implications since, at least at steady state, the toxic effects of local anaesthetics are directly related to the free (unbound) drug concentration. After injection into the epidural space, absorption into the bloodstream follows a biphasic process. The buffering properties of the epidural space are important and prevent a rapid rise in concentration. In infants and children, the epidural space seems to protect patients in a similar manner. Moreover, it has been observed that the peak plasma concentration (C(max)) of ropivacaine is delayed in infants and children when compared with adults. The time to C(max) decreases from 90-120 minutes in infants aged less than 6 months to 30 minutes in children aged more than 8 years. This delay in C(max) may also be related to the lower clearance observed in younger patients. Local anaesthetics are metabolised by cytochrome P450 (CYP). The main CYP isoforms involved are CYP3A4 for lidocaine and bupivacaine and CYP1A2 for ropivacaine. CYP3A4 is not mature at birth but is partly replaced by CYP3A7. The intrinsic clearance of bupivacaine is only one-third of that in adults at 1 month of age, and two-thirds at 6 months. CYP1A2 is not fully mature before the age of 3 years. Indeed, the clearance of ropivacaine does not reach its maximum before the age of 5 years. However, at birth this clearance is not as low as expected, and ropivacaine may be used even in younger patients.

Magnitude and Time-Course of Arterio-Venous Differences in Blood-Alcohol Concentration in Healthy Men

BACKGROUND AND OBJECTIVE

Human studies of arterio-venous (AV) differences in drug concentrations and the consequences for pharmacokinetic modelling and concentration-effect relationships are very limited. We therefore investigated the intravenous and intra-arterial concentrations of alcohol (ethanol) during the absorption, distribution and elimination stages of alcohol metabolism in healthy men.

STUDY PARTICIPANTS AND METHODS

Nine male volunteers aged 26-67 years drank 0.6 g alcohol/kg bodyweight in 2-15 minutes. The drink was prepared from 95% v/v alcohol, which was diluted with an alcohol-free beverage to 20% v/v. Before the start of drinking and for 6-7 hours post-administration, blood samples were drawn at 15- to 20-minute intervals from indwelling catheters in a radial artery and a cubital vein on the same arm. The blood-alcohol concentration (BAC) was determined by headspace gas chromatography, and blood-water content was measured by desiccation.

RESULTS

The peak concentration (Cmax) of alcohol in arterial blood was 0.98 g/L (SD 0.209) compared with 0.84 g/L (SD 0.176) for venous blood (p < 0.001), whereas median time to reach Cmax (tmax) was the same (35 minutes). The AV difference was greatest at 10 minutes after the end of drinking (mean 0.20 g/L [range 0.09-0.40 g/L]), decreasing as the absorption of alcohol continued. At a median time of 90 minutes post-administration (range 45-105 minutes), the AV difference was momentarily zero. At later times, the AV differences became increasingly negative and at 280 minutes post-administration the mean was -0.051 g/L (range -0.025 to -0.078 g/L). The slope of the post-absorptive phase (k0) was 0.116 g/L/h (SD 0.0167) for arterial blood compared with 0.109 g/L/h (SD 0.0185) for venous blood (p < 0.001). The extrapolated time to reach zero BAC was 391 minutes (SD 34) for arterial blood and 420 minutes (SD 41) for venous blood; the difference of 29 minutes was statistically highly significant (p < 0.001). The apparent volume of distribution of alcohol, the area under the concentration-time curves (AUC) and the water content of arterial and venous blood samples were not significantly different for the two sampling compartments.

CONCLUSION

The arterial and venous blood-alcohol profiles were shifted in time owing to the time it takes for alcohol to equilibrate between arterial blood and tissue water. Alcohol is metabolised in the liver but not in muscle tissue, which acts as a reservoir for alcohol. The concentrations of alcohol in arterial and venous blood were the same at only one timepoint, which signifies complete equilibration of alcohol in total body water. During the entire post-absorptive phase, the concentration of alcohol in venous blood draining skeletal muscles was slightly greater than the arterial blood concentration; therefore, the AV differences were negative.

Bone as an effect compartment : models for uptake and release of drugs

"Bone-seeking agents" are drugs characterised by high affinity for bone, and are disposed in bone for prolonged periods of time while maintaining remarkably low systemic concentrations. As a consequence, the bone becomes a reservoir for bone-seeking agents, and a site of both desirable and adverse effects, depending on the pharmacological activities of the specific agent. For some agents, significant systemic effects may also be produced following their prolonged release from bone, a process that is governed mostly by the rate of bone remodelling. This review covers the pharmacokinetic and pharmacodynamic features of bone-seeking agents with different pharmacological properties, including drugs (bisphosphonates, drug-bisphosphonate conjugates, radiopharmaceuticals and fluoride), bone markers (tetracycline, bone imaging agents) and toxins (lead, chromium, aluminium). In addition, drugs that do not possess bone-seeking properties but are used for therapy of bone diseases (such as antibacterials for treatment of osteomyelitis) are discussed, along with targeting of these drugs to the bone by conjugation to bone-seeking agents, local delivery systems, and other approaches. The pharmacokinetic and pharmacodynamic behaviour of bone-seeking agents is extremely complex due to heterogeneity in bone morphology and physiology. This complexity, accompanied by difficulties in human bone research caused by ethical and other limitations, gave rise to modelling approaches to study bone drug disposition. This review describes the pharmacokinetic models that have been proposed to describe the pharmacokinetic behaviour of bone-seeking agents and predict bone concentrations of these agents for different doses and patient populations. Models of different types (compartmental and physiologically based) and of different complexity have been applied, but their relevance to drug effects in the bone tissue is limited since they describe the behaviour of the "average" drug molecule. Understanding of the cellular and molecular processes responsible for the heterogeneity of bone tissue will provide better comprehension of the influence of microenvironment on drug bone disposition and the resulting pharmacological response.

Determinants of drug onset

PURPOSE OF REVIEW

The timing and magnitude of drug onset can be influenced by factors in the chain of drug delivery from the site of administration to the site of effect. This review examines recent evidence regarding the contribution and significance of these factors.

RECENT FINDINGS

It is apparent that drug formulations and mixtures can play a significant role in drug onset. An extension of this is the effect of coadministration of drugs, which can influence drug effect both by altering the physiology underlying drug delivery and by an effect at the target organ. Of the physiological variables, cardiac output and its distribution are clearly important. Cardiac output is a significant source of variability in drug response, and indeed has been successfully incorporated into pharmacokinetic models. The pattern of cardiac output distribution is also relevant. In particular, the blood flow to target organs will influence both the timing and magnitude of the effect of some anaesthetic drugs. In addition, the role of the lung in affecting drug distribution may be important for some drugs. At the site or organ of effect itself, variability in drug distribution, drug-receptor interactions, and the influence of other drugs, can all impact on the profile of drug onset.

SUMMARY

Factors in the chain of drug delivery have been demonstrated to affect the nature of drug onset, and can account for some of the observed variability in response. The significance to dosing guidelines and strategies, and to predictions of variability in response, remains to be explored.