Biological rhythms not only impact the pathophysiology of diseases, but the pharmacokinetics and pharmacodynamics of medications. Chronopharmacology is the investigative science that elucidates the biological rhythm dependencies of medications.
—Chronopharmacokinetics—
Chronopharmacokinetic studies have been reported for many drugs in an attempt to explain chronopharmacological phenomena and demonstrate that the time of administration is a possible factor of variation in the pharmacokinetics of a drug. Time-dependent changes in pharmacokinetics may proceed from 24 hr rhythms in each process, e.g. absorption, distribution, metabolism and elimination. Thus, 24 hr rhythms in gastric acid secretion and pH, motility, gastric emptying time, gastrointestinal blood ‰ow, drug protein binding, liver enzyme activity andWor hepatic blood ‰ow, glomerular ˆltration, renal blood ‰ow, urinary pH and tubular resorption may play a role in such pharmacokinetic variations.4)
The clock genes are expressed not only in the SCN, but also in other brain regions and various peripheral tissues. A microarray analysis experiment has revealed that there are many genes expressing a circadian rhythm in the liver.26) The liver is a biological clock capable of generating its own circadian rhythms.27) Since the liver is a major organ of metabolism and detoxiˆcation, knowledge of circadian eŠects on transcriptional activities that govern daily biochemical and physiological processes in the liver may play a key role in toxicology. Relative levels of gene expression in the liver of rats is investigated from the viewpoint of time of day.26) Expression levels are determined for 3906 genes using highdensity oligonucleotide microarrays. Of them 30z are clearly expressed while 70z are not expressed or the expression is very low. The maximum estimated changes observed for most genes (90z) of rhythmic genes are less than 1.5-fold. 67 genes show signiˆcant rhythmic expression. These altered genes include DNA binding and regulation of transcription, drug metabolism, ion transport, signal transduction and immune response. A circadian rhythm is demonstrated for six genes involved in regulation of gene transcription.26) The retinoic acid receptor-alpha and the retinoid X receptors (alpha and gamma) play an important role in regulation of gene expression by forming transcriptionally active complexes on DNA. Aryl hydrocarbon receptor nuclear translocator (Arnt) works as a transcription factor in diverse signaling events including response to xenobiotics. A circadian expression is also demonstrated for Pitx2 and Pitx3 genes. These genes encode paired-like homeodomain transcription factors 2 and 3, the members of homeobox gene family. Drug metabolism is the main function of the liver. There is a signiˆcant circadian rhythm in cytochrome P-450 4a3 (Cyp4a3) and putative N-acetyltransferase camello 4 (Clm4) of phase I and phase II of drug metabolism.26) Liver cytochrome P450 4a isoforms play an important role in regulation of renal function by catalyzing the formation of 20- hydroxyeicosatetraenoic acid. This may be one of mechanisms underlying circadian rhythm of renal function and blood pressure. In mouse liver, circadian regulation of transcripts is demonstrated for the cytochrome P450 such as Cyp17, Cyp2a4, Cyp2e1, Cyp2c22 and so on. Clm4 encodes the protein catalyzing acetylation of aromatic amines and hydrazines. The rhythmic pattern nicely corresponds to the pattern of Cml2 in mouse liver. Circadian rhythm is demonstrated for other members of the phase II of drug metabolism such as glutathione S-transferases (GST) and carboxylesterase. The liver is the major organ of metabolism and endures a ‰ux of metabolites across membranes. A signiˆcant circadian rhythm is demonstrated for genes involved in ion transport.26) They include genes encoding proteins of the solute carrier transporter such as Slc34a1 and Slc2a8. A rhythmic gene expression is demonstrated for solute carriers such as Slc12a2, Slc16a1, Slc19a1, and Slc25a11. In addition to the anion and solute transporters Abcc2 and Aqp9, expressions of Slc10a1, Slc22a1, Slc27a1, Slc2a2, and Slc7a2 show circadian rhythm. Furthermore, there is a signiˆcant circadian expression of ion transporter genes such as Hcn4, Trpc4, Scn2b, Scn4a, Chrnb2, Atp9a, Atp7b, Timm10, and Nritp. Since one of the important defense mechanisms includes the active extrusion of xenobiotics by transporter, genes involved in ion or solute transport activity may have signi ˆcant implications in toxicology studies. Coordinated rhythmic oscillations in phase I and phase II components of drug metabolism during the day may explain diŠerential responses to drugs in toxicology. DBP is able to activate the promoter of a putative clock oscillating gene, Per1, by directly binding to the Per1 promoter.13,28,29) The Per1 promoter is cooperatively activated by DBP and CLOCK-BMAL1. On the other hand, Dbp transcription is activated by CLOCKBMAL1 through E-boxes and inhibited by the PER and CRY proteins, as is case for Per1. Thus, Dbp, a clockcontrolled gene whose expression oscillates with a very high circadian amplitude, may play an important role in central clock oscillation. Dbp participates in the regulation of several clock outputs, including locomotor activity, sleep distribution, and liver gene expression. Also, DBP is a major factor controlling circadian expression of the steroid 15 a-hydroxylase (Cyp3a4) and coumarin 7-hydroxylase (Cyp2a5) genes in mouse liver.30) Thus, the mechanisms underlying 24 hr rhythm of drug metabolism have been gradually clariˆed. A signiˆcant portion of the transcriptome in mammals, including the PAR-domain basic leucine zipper (PAR bZip) transcription factors DBP, HLF, and TEF, is controlled by circadian clock. Triple mutant mice are epilepsy prone, age at an accelerated rate, and die prematurely.31) The PAR bZip transcription factors DBP, TEF, and HLF accumulate in a highly circadian manner in several peripheral tissues such as liver and kidney. To identify PAR bZip target genes whose altered expression will contribute to the high morbidity and mortality of PAR bZip triple knockout mice, the liver and kidney transcriptomes of these animals are compared with those of wild-type or heterozygous mutant mice. The disruption of these three genes in mice alters gene expression patterns of many proteins involved in drug metabolism and transporter. The various levels at which PAR bZip transcription factors might intervene in the coordination of xenobiotic detoxiˆcation are described in Fig. 4. The PAR bZip proteins control the expression of many enzymes and regulators involved in detoxiˆcation and drug metabolism, such as cytochrome P450 enzymes, carboxylesterases, aminolevulinic acid synthase (ALAS1), P450-oxidoreductase (POR), sulfotransferases, GST, aldehyde dehydrogenases, UDP-glucuronosyltransferases, members of drug transporter families, and constitutive androstane receptor (CAR). Some genes encoding detoxiˆ- cation enzymes such as CYP2A5, CYP2C50, CES3 may be direct PAR bZip target genes. The expression of other detoxiˆcation enzymes such as CYP2B10, is mostly regulated by CAR, whose circadian transcription is governed by PAR bZip proteins. Other enzymes in the xenobiotic defence such as ALAS1 and POR appear to be controlled by both CAR and PAR bZip proteins. Rhythmic changes in transcriptional regulators will be further analyzed in future studies. Clock genes play a key role in the molecular clockworks of both the SCN and the liver. Although oscillation of clock genes in the liver is controlled by the circadian clock mechanism in the SCN, the resetting signals on liver clock function have not been clariˆed yet. A transsynaptic tract tracer using the pseudorabies virus has clariˆed neural connections between the SCN and peripheral tissues. Communication between the SCN and peripheral tissues occurs through autonomic nervous systems. Although further study is necessary to clarify the mechanism underlying neural control of liver clock systems, evolution of this mechanism will be useful to the understanding of liver clock functions such as drug metabolism and energy metabolism.
—Chronopharmacodynamics—
Biological rhythms at the cellular and subcellular level can give rise to signiˆcant dosing-time diŠerences in the pharmacodynamics of medications that are unrelated to their pharmacokinetics. This phenomenon is termed chronesthesy. Rhythms in receptor number or conformation, second messengers, metabolic pathways, andW or free-to-bound fraction of medications help explain this phenomenon. For example, the antitumor eŠect of IFN-b and the antiviral eŠect and lymphocyte stimulating eŠect of IFN-a in mice are more e‹cient during the early rest phase than during the early active phase.32,33) The dosing schedule-dependent eŠect of IFN-b or IFN-a is also closely related to that of IFNs receptors and ISGF expression in tumor cells or lymphocytes. The term chronotoxicity refers speciˆcally to predictable- in-time variation in patient vulnerability to the side eŠects of medications due to biological rhythm determinants. Chronotoxicities are known especially with antitumor agents. For example, the body weight loss with irinotecan hydrochloride (CPT-11) of nocturnally active mice is more serious in the late active phase and the early rest phase and milder in the late rest phase and the early active phase.34) CPT-11-induced leukopenia is more serious in the late active phase and milder in the late rest phase. The lower toxicity of CPT-11 is observed when DNA synthesis and type I DNA topoisomerase activity in bone marrow cells decrease and higher toxicity is observed when these activities begin to increase. The ˆnding indicates that the choice of dosing time associated with the 24 hr rhythm of DNA synthesis may help to achieve a rational chronotherapeutic strategy, reducing the toxic eŠects of CPT-11 andWor increasing its therapeutic eŠects. Cell division in many mammalian tissues is associated with speciˆc times of day. In the regenerating liver of mice, the circadian clock controls the expression of cell cyclerelated genes that in turn modulate the expression of active Cyclin B1-Cdc2 kinase, a key regulator of mitosis. 35) Among these genes, expression of wee1 is directly regulated by the molecular components of the circadian clockwork. On the other hand, the circadian clockwork oscillates independently of the cell cycle in single cells. The intracellular circadian clockwork can control the cell-division cycle directly and unidirectionally in proliferating cells. Thus, the regulatory mechanisms underlying 24 hr rhythm of pharmacodynamics should be also clariˆed from viewpoints of clock genes. Angiogenesis is important for tumor growth and metastasis. Hypoxia-induced expression of vascular endothelial growth factor (VEGF) plays an important role in tumor-induced angiogenesis. The levels of VEGF mRNA in tumor cells implanted in mice rise substantially in response to hypoxia, but the levels show a 24 hr rhythm.36) Luciferase reporter gene analysis reveals that PER2 and CRY1, whose expression in the implanted tumor cells shows a 24 hr rhythm, inhibit the hypoxiainduced VEGF promoter activity. Namely, the negative limbs of the molecular loop periodically inhibit the hypoxic induction of VEGF transcription, resulting in the 24 hr ‰uctuation of its mRNA expression. Furthermore, the antitumor e‹cacy of antiangiogenic agents is enhanced by administering the drugs at the time when VEGF production increases.
Methionine aminopeptidase2 (MetAP2) plays an important role in the growth of endothelial cells during the tumor angiogenesis stage. MetAPs show a 24 hr rhythm in implanted tumor masses.37) The mechanism underlying the 24 hr rhythm of MetAP2 activity is investigated in tumor-bearing mice. The 5? ‰anking region of MetAP2 includes eight E-boxes. The transcription of the MetAP2 promoter is enhanced by the CLOCK: BMAL1 heterodimer, and its activation is inhibited by PER2 or CRY1. Deletion and mutation of the E-boxes in the region indicates that the E-box nearest to the initiation start site plays an important role in the transcriptional regulation by clock genes. In sarcoma180-bearing mice, the pattern of binding of CLOCK and BMAL1 to the E-box and transcription of the MetAP2 promoter shows a 24 hr rhythm with higher levels from the midlight to early dark phase. MetAP2 protein expression varies with higher levels from the late-dark to early-light phase. Namely, the 24 hr rhythm of MetAP2 activity is regulated by the transcription of clock genes within the clock feedback loops. Furthermore, the antitumor e‹cacy of MetAP2 inhibitor is enhanced by administering the drugs at the time when MetAP2 activity increases.
—Chrono-Drug Delivery System (chrono-DDS)—
The eŠectiveness and toxicity of many drugs vary depending on the 24 hr rhythms of biochemical, physiological and behavioral processes. Also, several drugs can cause alterations to the 24 hr rhythms leading to illness and altered homeostatic regulation. The alteration of biological rhythm is a new concept of adverse eŠects. It can be minimized by optimizing the dosing schedule.38) Many researches demonstrate the rationale behind chronotherapy;39) however, drug delivery research has focused on a constant drug release rate. The reason why the majority of DDS is designed without emphasis on proven oscillatory phenomena may be in drug delivery limitations. Advances in chronobiology and global market constraints changes the traditional goal of pharmaceutics such as a constant drug release rate. The increasing research interest on Chrono-DDS may create a new sub-discipline in chronopharmaceutics. The technologies in chronopharmaceutics includes: CONTIN}, physicochemical modiˆcation of the active pharmaceutical ingredient, OROS}, CODAS}, CEFORM}, DIFFUCAPS}, chronomodulating infusion pumps, TIMERx}, three-dimensional printing, controlled-release erodible polymer and controlledrelease microchip strategies.39) As examples of Chrono- DDS on the market, there are compounds such as theophylline (Uniphyl}), famotidine (Pepcid}), simvastatin (Zocor}), COER-verapamil (Covera-HS}, Verelan} PM), diltiazem (Cardizem} LA) and propranolol (InnoPran} XL).39) Most data have been compiled from the FDA electronic orange book,40) speciˆc product package inserts and United States patents and speciˆc pharmaceutical company websites. Future development in chronopharmaceutics may be performed by the new technology such as system biology and nanomedicine.
Answers to all your Biology Questions
