Clinical pharmacokinetic and pharmacodynamic properties of drugs used in the treatment of Parkinson's disease

Current research in Parkinson's disease (PD) focuses on symptomatic therapy and neuroprotective interventions. Drugs that have been used for symptomatic therapy are levodopa, usually combined with a peripheral decarboxylase inhibitor, synthetic dopamine receptor agonists, centrally-acting antimuscarinic drugs, amantadine, monoamine oxidase-B (MAO-B) inhibitors and catechol-O-methyltransferase (COMT) inhibitors. Drugs for which there is at least some evidence for neuroprotective effect are certain dopamine agonists, amantadine and MAO-B inhibitors (selegiline). Levodopa remains the most effective drug for the treatment of PD. Several factors contribute to the complex clinical pharmacokinetics of levodopa: erratic absorption, short half-life, peripheral O-methylation and facilitated transport across the blood-brain barrier. In patients with response fluctuations to levodopa, the concentration-effect curve becomes steeper and shifts to the right compared with patients with stable response. Pharmacokinetic-pharmacodynamic modelling can affect decisions regarding therapeutic strategies. The dopamine agonists include ergot derivatives (bromocriptine, pergolide, lisuride and cabergoline), non-ergoline derivatives (pramipexole, ropinirole and piribedil) and apomorphine. Most dopamine agonists have their specific pharmacological profile. They are used in monotherapy and as an adjunct to levodopa in early and advanced PD. Few pharmacokinetic and pharmacodynamic data are available regarding centrally acting antimuscarinic drugs. They are characterised by rapid absorption after oral intake, large volume of distribution and low clearance relative to hepatic blood flow, with extensive metabolism. The mechanism of action of amantadine remains elusive. It is well absorbed and widely distributed. Since elimination is primarily by renal clearance, accumulation of the drug can occur in patients with renal dysfunction and dosage reduction must be envisaged. The COMT inhibitors entacapone and tolcapone dose-dependently inhibit the formation of the major metabolite of levodopa, 3-O-methyldopa, and improve the bioavailability and reduce the clearance of levodopa without significantly affecting its absorption. They are useful adjuncts to levodopa in patients with end-of-dose fluctuations. The MAO-B inhibitor selegiline may have a dual effect: reducing the catabolism of dopamine and limiting the formation of neurotoxic free radicals. The pharmacokinetics of selegiline are highly variable; it has low bioavailability and large volume of distribution. The oral clearance is many-fold higher than the hepatic blood flow and the drug is extensively metabolised into several metabolites, some of them being active. Despite the introduction of several new drugs to the antiparkinsonian armamentarium, no single best treatment exists for an individual patient with PD. Particularly in the advanced stage of the disease, treatment should be individually tailored.     

Impact of genetic polymorphisms of transporters on the pharmacokinetic, pharmacodynamic and toxicological properties of anionic drugs

As the importance of drug transporters in the clinical pharmacokinetics of drugs is recognized, genetic polymorphisms of drug transporters have emerged as one of the determinant factors to produce the inter-individual variability of pharmacokinetics. Many clinical studies have shown the influence of genetic polymorphisms of drug transporters on the pharmacokinetics and subsequent pharmacological and toxicological effects of drugs. The functional change in a transporter in clearance organs such as liver and kidney affects the drug concentration in the blood circulation, while that in the pharmacological or toxicological target can alter the local concentration at the target sites without changing its plasma concentration. As for the transporters for organic anions, some single nucleotide polymorphisms (SNPs) or haplotypes occurring with high frequency in organic anion transporting polypeptide (OATP) 1B1, multidrug resistance 1 (MDR1), and breast cancer resistance protein (BCRP) have been extensively investigated in both human clinical studies and in vitro functional assays. We introduce some examples showing the relationship between haplotypes in transporters and pharmacokinetics and pharmacological effects of drugs. We also discuss how to predict the effect of functional changes in drug transporters caused by genetic polymorphisms on the pharmacokinetics of drugs from in vitro data.     

Individualized dosage regimen of immunosuppressive drugs based on pharmacokinetic and pharmacodynamic analysis

The calcineurin inhibitors cyclosporine and tacrolimus are widely used to prevent allograft rejection after transplantation. Since these drugs have narrow therapeutic windows and show considerable pharmacokinetic variability, therapeutic drug monitoring (TDM) is essential to avoid adverse effects such as nephrotoxicity while maximizing immunosuppressive efficacy. On the other hand, some patients experience acute rejection episodes or postoperative complications despite achieving therapeutic blood drug levels. Therefore, pharmacokinetic and pharmacodynamic factors by which to establish individualized dosage adjustment for these drugs should be identified. Recently, it was recognized that pharmacogenomics has the potential to facilitate personalized medicine by translating knowledge of human genome variability into rational therapeutics. In this paper, we review the population pharmacokinetic and pharmacogenomic analysis of tacrolimus, focusing on an efflux transporter P-glycoprotein (multidrug resistance 1 [MDR1/ABCB1]) and drug-metabolizing enzymes cytochrome P450 (CYP) 3A4 and 3A5, and describe Bayesian forecasting to individualize the tacrolimus dose in de novo living-donor liver transplant recipients. Furthermore, the pharmacodynamic properties of tacrolimus and cyclosporine, which were evaluated by measuring calcineurin phosphatase activity in peripheral blood mononuclear cells, are reviewed in relation to an optimal monitoring strategy as well as a rational dosage regimen for these drugs.     

Ocular pharmacokinetic/pharmacodynamic modeling for multiple anti-glaucoma drugs

We have constructed a new ocular pharmacokinetic pharmacodynamic (PK/PD) model for anti-glaucoma drugs to describe ocular hypotensive effects on intraocular pressure (IOP) after instillation of a combination of an alpha(1)-adrenergic antagonist, bunazosin, and a beta-adrenergic antagonist, timolol, into rabbits. This model was constructed by the combination of two ocular PK/PD models for bunazosin and timolol by including aqueous humor dynamics based on both action mechanisms. We also verified the reliability of this model by confirming the drug concentrations in aqueous humor and ocular hypotensive effects after instillation of the drug combination. The aqueous humor concentrations of timolol and bunazosin were determined by an HPLC, and ocular hypotensive effect-time profiles were measured using a telemetry system, which was able to record automatically detailed effects. The combined model could simulate the aqueous humor concentrations of both drugs and the additive IOP-lowering effect after instillation of the combination using the MULTI (RUNGE) program and PK/PD parameters which were obtained from ocular hypotensive effects after instillation of bunazosin alone or timolol alone. The theoretical concentration curves of both drugs in the aqueous humor and the theoretical ocular hypotensive effect curves almost agreed with both the observed concentrations and ocular hypotensive effects after instillation of the drug combination. These results indicate the reliability and usefulness of PK/PD modeling considering aqueous humor dynamics to predict IOP in multidrug therapy. This is the first study to develop a PK/PD model for multidrug therapy for the eye.     

Species differences in pharmacokinetic and pharmacodynamic properties of nebicapone

The present study was designed to characterize pharmacodynamic and pharmacokinetic properties of nebicapone in rats and mice. Upon oral administration of nebicapone the extent of mouse liver catechol-O-methyltransferase (COMT) inhibition is half that in the rat. Nebicapone was rapidly absorbed reaching plasma C_max within 30 min and being completely eliminated by 8 h. Nebicapone was metabolized mainly by glucuronidation and methylation in both species, but rat had an extra major metabolite, resulting from sulphation. Administration of nebicapone by the intraperitoneal route significantly increased compound AUC in the rat while in the mouse a significant increase in AUC of metabolites was observed. These results show that nebicapone exhibited marked species differences in bioavailability and metabolic profile. Evaluation of COMT activity in rat and mice liver homogenates revealed that both had similar methylation efficiencies (K_cat values, respectively 7.3 and 6.4 min⁻¹), but rat had twice active enzyme units as the mouse (molar equivalency respectively 150 and 83). Furthermore, nebicapone inhibited rat liver COMT with a lower K_i than mouse liver COMT (respectively 0.2 nM vs. 1.2 nM). In conclusion, the results from the present study show that mice and rats respond differently to COMT inhibition by nebicapone. The more pronounced inhibitory effects of nebicapone in the rat may be related to an enhanced oral availability and less pronounced metabolism of nebicapone in this specie, but also concerned with the predominant expression of S-COMT over MB-COMT, the latter of which is less sensitive to inhibition by nebicapone than the former.     

Amphiphilic drugs. 2. Relation between colloidal properties and pharmaceutic-technological, pharmacokinetic and pharmacodynamic behavior

The physicochemical properties of amphiphilic drugs may affect the technological, pharmacokinetic and pharmacodynamic behaviours. Micelle formation of a drug is of considerable importance for its solubility, stability, adsorption pattern, mixed micelle formation and influences thus its pharmaceutical availability. The biological availability is in many cases decreased by interactions of a surface active drug with other substances before absorption, by altered diffusion behaviour and distribution properties. Colloidal association may also result in an alteration of pharmacodynamic effects. These relations depend on the fact, that discrete hydrophobic an hydrophilic parts in a molecule are also prerequisites for micelle formation and receptor interactions. Furthermore the high membrane affinity of these drugs is responsible for some therapeutic and toxic effects.     

Effects of sex on the pharmacokinetic and pharmacodynamic properties of quinidine

AIMS: To investigate the source of the apparent increased susceptibility of women to develop QT interval prolongation and torsade de pointes after the administration of drugs that delay cardiac repolarization. METHODS: Plasma quinidine concentrations and electrocardiographic changes (QRS and QT intervals) were measured over 24 h following the administration of single oral doses of the QT prolonging drug quinidine (3 mg kg(-1)) and compared between 27 male and 21 female healthy volunteers. RESULTS: There were no significant differences between males and females in plasma quinidine concentrations or in calculated pharmacokinetic variables. Maximum quinidine concentrations in males and females were 997 +/- 56 and 871 +/- 57 ng ml(-1), respectively (mean difference (-125, 95% confidence intervals (CI) -239, 11 ng ml(-1), P = NS). Quinidine lengthened actual (QTa) and corrected (QTc) QT intervals and the QRS interval to a greater extent in females than males (P < 0.001 for each), but there were no significant sex differences detected in the effects of quinidine on the heart rate corrected JT interval. Maximum prolongation of QTc interval was observed 2 h after quinidine and was significantly greater in women (33 +/- 16 vs 24 +/- 17 ms, mean difference 9 +/- 20 ms, 95% CI 3, 15, P = 0.037). At this time mean differences (95% CI) were 1.0 min(-1) (-2.5, 4.4, P = NS) for heart rate, 5.5 ms (3.5, 7.6, P = 0.05) for the QRS and 3.4 ms (-2.5, 9.3, P = NS) for the JTc intervals. CONCLUSIONS: Quinidine-induced increases in QTc were larger in females, but no sex differences in quinidine pharmacokinetics were found. The disparity in prolongation of cardiac repolarization is thus due to a pharmacodynamic difference which appears more complex than simply an increase in repolarization delay in females.     

Moclobemide: evolution,pharmacodynamic, and pharmacokinetic properties

The benzamide moclobemide is a reversible inhibitor of monoamine-oxidase-A (RIMA). It has been extensively evaluated in the treatment of a wide spectrum of depressive disorders and less extensively in anxiety disorders. While clinical aspects will be presented in a subsequent review, this article focuses primarily on moclobemide's evolution, pharmacodynamic and pharmacokinetic properties. In particular, the effects on neurotransmission and intracellular signal transduction, the neuroendocrine system, the tyramine pressure response and animal models of depression are surveyed. In addition, other CNS effects are reviewed with special respect to experimental serotonergic syndrome, anxiolytic and antinociceptive activity, sleep, cognition and driving performance, neuroprotection and seizures.     

Animal-to-human extrapolation of the pharmacokinetic and pharmacodynamic properties of buprenorphine

OBJECTIVES: This investigation describes the interspecies scaling of the pharmacokinetics and pharmacodynamics of buprenorphine. METHODS: Data on the time course of the antinociceptive and respiratory depressant effects of buprenorphine in rats and in humans were simultaneously analysed on the basis of a mechanism-based pharmacokinetic-pharmacodynamic model. RESULTS: An allometric three-compartment pharmacokinetic model described the time course of the concentration in plasma. The value of the allometric coefficient for clearance was 35.2 mL/min (relative standard error [RSE] = 5.6%) and the value of the allometric exponent was 0.76 (RSE 5.61%). A combined biophase distribution-receptor association/dissociation model with a linear transduction function described hysteresis between plasma concentration and effect. The values of the drug-specific pharmacodynamic parameters were identical in rats and in humans. For the respiratory depressant effect, the values of the second-order rate constant of receptor association (k(on)) and the first-order rate constant of receptor dissociation (k(off)) were 0.23 mL/ng/min (RSE = 15.8%) and 0.014 min(-1) (RSE = 27.7%), respectively, and the value of the equilibrium dissociation constant (K(diss)) was 0.13 nmol/L. The value of the intrinsic activity alpha was 0.52 (RSE = 3.4%). For the antinociceptive effect, the values of the k(on) and k(off) were 0.015 mL/ng/min (RSE = 18.3%) and 0.053 min(-1) (RSE = 23.1%), respectively. The value of the K(diss) was 7.5 nmol/L. An allometric equation described the scaling of the system-specific parameter, the first-order distribution rate constant (k(e0)). The value of the allometric coefficient for the k(e0) was 0.0303 min(-1) (RSE = 11.3%) and the value of the exponent was -0.28 (RSE = 9.6%). CONCLUSIONS: The different values of the drug-specific pharmacodynamic parameters are consistent with the different opioid mu receptor subtypes involved in the antinociceptive and respiratory depressant effects.     

Relationship of pharmacokinetic and pharmacodynamic properties of the organic nitrates

Glyceryl trinitrate (nitroglycerin), isosorbide dinitrate and isosorbide mononitrate are, in various formulations, available for clinical use. The pharmacokinetics of glyceryl trinitrate are complex and only 1% of the drug administered orally can be detected in the plasma due to a pronounced arteriovenous concentration gradient, hydrolysis in the blood, and rapid denitration in the liver. There is a poor and usually transient correlation between plasma concentrations and therapeutic effects, due in part to the complex pharmacokinetics of glyceryl trinitrate, but primarily due to development of tolerance during continuous administration, either via the intravenous or cutaneous route. Isosorbide dinitrate kinetics are complicated by its extensive metabolism into 2- and 5-mononitrates, which are pharmacologically active, and which also accumulate more than the parent drug during long term treatment. These facts, combined with development of tolerance during long term therapy, preclude the establishment of a concentration-response relationship. Isosorbide-5-mononitrate has ideal and dose-linear kinetics and is nearly 100% bioavailable after oral administration. However, tolerance develops during long term therapy, and therefore a relationship between plasma concentrations and clinical effects cannot be established. On the basis of available data, plasma concentrations of various nitrates do not reliably predict clinical effects.     

Gliclazide. A preliminary review of its pharmacodynamic properties and therapeutic efficacy in diabetes mellitus

Gliclazide is a 'second generation' oral hypoglycaemic agent. The particular interest with this drug is that it has shown certain effects on the blood for which it is hoped there may be some clinical benefit in diabetic angiopathies. Both in animal and human studies it has demonstrated a reduction in platelet adhesiveness and aggregation, whilst possible enhancement of platelet metabolism, reduction of coagulant factors, as well as increased fibrinolytic activity, are still being investigated. Initial trials have suggested that gliclazide therapy may reverse or at least slow down the progression of diabetic retinopathy. However, a few additional well-designed long term controlled studies are needed to confirm these findings, and to clarify whether any beneficial effect on diabetic retinopathy is unique to gliclazide or also occurs with other oral hypoglycaemic drugs. Both newly diagnosed maturity onset diabetics as well as those previously treated with sulphonylureas respond well to gliclazide therapy. In the small comparative studies which have been reported, gliclazide was of comparable efficacy to other oral hypoglycaemic agents.     

Microdialysis for in vivo pharmacokinetic/pharmacodynamic characterization of anti-infective drugs

Inadequate tissue penetration of antibiotics can lead to therapeutic failure and bacterial resistance. Pharmacokinetic evaluation of antibiotics should therefore be based on tissue rather than serum concentrations. Over several years, tissue concentration data obtained by methods such as tissue biopsies have flawed the correct interpretation of antibiotic tissue distribution. Microdialysis--a semi-invasive catheter-based sampling technique--has been employed for the in vivo measurement of antibiotic tissue pharmacokinetics. Owing to selective access to the target site for most anti-infective drugs, microdialysis satisfies regulatory requirements for pharmacokinetic distribution studies and might become a reference technique for tissue distribution studies in the near future. Furthermore, microdialysis might contribute to the definition of meaningful surrogate markers for antibiotic efficiency during drug development.     

Genetic influences on pharmacodynamic properties of psychotropic drugs.

Most of the well-known examples of pharmacogenetics are based on differences of biotransformation and elimination of drugs. Since a drug interacts with a genetically determined biological target, hereditary differences on the pharmacodynamic level are also probable. Especially psychotropics show interindividual variation in their pharmacological effects. The differential effects of psychotropic drugs on the EEG presumably reflect genetic differences of brain function. Lithium transport across the erythrocyte membrane exhibits large interindividual differences that have a genetic basis, and that possibly indicate a link between lithium transport and manic-depressive psychosis.     

Studies of pharmacokinetic and pharmacodynamic properties of isoallopregnanolone in healthy women

Rationale  The pharmacokinetics and behavioral effects of isoallopregnanolone (3β-hydoxy-5α-pregnan-20-one) in women are not known. Objectives  Allopregnanolone (3α-hydoxy-5α-pregnan-20-one) is a well-known neurosteroid, acting via the GABA_A receptor in the human brain. The naturally occurring progesterone metabolite isoallopregnanolone is the 3β-stereoisomer of allopregnanolone. Prior studies have concluded that isoallopregnanolone has no effect on the GABA_A receptor. However, an antagonistic effect of isoallopregnanolone to allopregnanolone on the GABA_A receptor has been shown in animal and in vitro studies. The purpose of this study was to evaluate the pharmacokinetics and behavioral effects of isoallopregnanolone in humans. Materials and methods  Six healthy women were given three increasing doses of isoallopregnanolone intravenously in the follicular phase. Repeated blood samples for analyses of isoallopregnanolone and allopregnanolone concentrations were drawn. Saccadic eye movement variables, self-rated sedation, and mood rating scales were used during the test day. A Likert scale for prospective symptoms was used to measure daily fluctuations during the ongoing menstrual cycle. Results  Exogenously administered isoallopregnanolone produced a dose-dependent increase in the serum concentration of isoallopregnanolone. In parallel, there was also a rise in the allopregnanolone concentration. There was a decrease in saccadic eye movement variables, but no effect was found on self-rated sedation or mood and no changes were seen in prospective symptoms during the menstrual cycle. Conclusions  After administration of isoallopregnanolone at a cumulative dose of 0.20 mg/kg, no adverse effects were observed. There is a metabolism of isoallopregnanolone to allopregnanolone, most likely explaining the effects on the saccadic eye movements.     

Methadone: a review of its pharmacokinetic/pharmacodynamic properties

During the past decades the use of methadone has been increased as a result of the interest of optimizing its therapeutics in opioid addicts, one of the groups with higher risk for AIDS infection. However standard dose of methadone are far from being the appropriate for relief pain or prevent withdrawal signs in maintenance programs in many patients. To achieve an optimal dose regimen for an individual, the knowledge of the relationship between the pharmacokinetics/pharmacodynamics (pk/pd) drug properties and the demographic and physiopathological characteristics of the subject is required. Unfortunately, there is a lack of studies dealing with the population pk/pd properties of methadone. In the current study, a review of the pk/pd properties of methadone is presented with the aim of understanding the sources of variability in response. This will help in the design of prospective pk/pd studies; in particular, individual data including sex, weight, alpha(1)-acid glycoprotein levels in plasma, concomitant medications, time after starting treatment with methadone and previous exposure to other opioids should be requested. In addition, designs for drug administration should allow the characterization of the plasma-versus-biophase distribution and the development of tolerance processes. Because methadone is usually administered as a racemic mixture, the use of enantioselective techniques to determine both enantiomers in plasma is also highly recommended.     

Simultaneous pharmacokinetic and pharmacodynamic modeling

The compartmental model approach previously proposed by Sheiner et al.for the simultaneous characterization of the pharmacokinetics and pharmacodynamics of a drug substance has been rigorously tested and extended in the present report. Pharmacokinetic and pharmacodynamic equations were derived for several commonly used models, as well as alternate models that relate effect to drug concentrations in a peripheral compartment. The versatility and flexibility of these models were tested using simulation and curve-fitting procedures. The utility as well as the limitations of this modeling procedure are discussed.     

Recent pharmacodynamic and pharmacokinetic findings on oxaprozin

Oxaprozin has proven a safe and effective treatment for arthritic conditions. It has a low incidence of gastrointestinal side effects making it a particularly attractive therapy. Being an unselective COX-1 and COX-2 inhibitor the drug may have advantages in more effective pain relief related to both COX-1 as well as COX-2 effects. We have investigated the pharmacology of oxaprozin on (a) the biochemical components of joint destruction in vitro, and (b) the pharmacokinetic interactions of this drug with albumin in vitro in which drug interactions are modelled by competitive binding of the drug with endogenous ligands or drugs that are commonly prescribed with NSAIDs. The latter in vitro pharmacokinetic study can be considered as a basis for understanding both safety and therapeutic activity of the drug. Among the major effects of oxaprozin (10-100 M) observed on components of joint destruction was (a) the inhibition of the production of interleukin-1 (IL-1) from pig synovial tissues in organ culture and IL-1, IL-6 and IL-8, as well as tumour necrosis factor- (TNF) from THP-1 mononuclear cells at 50-100 M. Oxaprozin (1.0-100 M) did not affect NO production from porcine synovial tissue, whereas indomethacin and nimesulide reduced production of NO. Oxaprozin did not exacerbate the IL-1 and/or TNF-induced proteoglycan destruction in pig or bovine cartilage in organ culture as observed with indomethacin and aspirin. Radiolabelled oxaprozin accumulated in cartilage to a much greater extent than observed with other NSAIDs. This may be a particular advantage in enabling expression of cartilage protective effects of the drug. Albumin binding of [¹⁴C]oxaprozin (10 M) in vitro was unaffected by other NSAIDs (e.g., aspirin, diclofenac, ibuprofen, paracetamol, salicylate), a range of commonly co-prescribed drugs (e.g., atenolol, clonidine, cromolyn, diphenhydramine, furosemide, ketotifen, salbutamol, prednisolone, theophylline), endogenous steroids (e.g., oestradiol, progesterone) or other agents (e.g., caffeine) at concentrations of 40 M. The free concentrations of oxaprozin were, however, increased slightly by the same concentrations of warfarin, prednisolone, diazepam and captopril. In contrast, the binding of oxaprozin to free-fatty acid depleted albumin was only appreciably affected by captopril and caffeine. The free oxaprozin was slightly increased by zinc and copper ions (which are increased in rheumatic conditions) in normal as well as fatty acid-free albumin. Tryptophan release from albumin, which has a role in central analgesic actions of anti-rheumatic drugs by enhancing CNS turnover of 5-hydroxytryptamine (serotonin), was increased by 50-100 M oxaprozin, as well as other NSAIDs. This indicates that serotonergic activation could also contributed to the analgesic properties of oxaprozin like that of other NSAIDs. These studies indicate that the potentially-significant pharmacodynamic actions of oxaprozin in arthritic joints may involve inhibition of the release of pro-inflammatory cytokines from inflamed synovium and relatively high rate of drug uptake into inflamed joints. Analgesia, in addition to being a consequence of prostaglandin inhibition, may be initiated by release of albumin-bound tryptophan and uptake into the brains leading to activate serotonergic pathways of pain suppression. Oxaprozin is only slightly affected pharmacokinetically by some co-prescribed drugs or patho-physiologic metal ions but these effects may be of limited clinical consequence principally as oxaprozin has a relatively long plasma elimination half-life (50-60 h) and any small increase in free concentrations of the drug in circulating plasma increases elimination of the drug.     

Utilisation of drugs for diabetes mellitus

This article reviews the data from drug utilisation research on antidiabetic agents, oral hypoglycaemics and insulin. Study methods specific to this type of pharmacoepidemiological research are discussed and critiqued. A brief overview of the sources of drug utilisation data is presented, followed by a review of specific pharmacoepidemiological investigations. We evaluate the usefulness of these studies in assessing true drug consumption, in evaluating comorbidity in diabetic patients and in measuring the prevalence of diabetes. International comparisons of antidiabetic drug utilisation, also reviewed and analysed, demonstrate wide variations in the use of hypoglycaemic agents, which have arisen for reasons which are unclear. Drug utilisation research thus far has been limited by the paucity of studies relating these variations in antidiabetic drug use to specific clinical outcomes. There is a need to expand the applications of research on the use of antidiabetic agents, including assessment of patterns of morbidity across geographic boundaries and over time.