Importance of Drug Pharmacokinetics at the Site of Action

Pharmacological activity depends on adequate drug levels at the effect site, but access to these sites for pharmacokinetic sampling is often limited. Thus, we often rely on plasma concentrations as surrogates in efforts to understand exposure–response relationships. Recent advances in analytical and quantitative methodologies allow for more robust and often noninvasive assessments of drug pharmacokinetics at the site of action. This review highlights the advances made in estimating drug exposures in several compartments of interest. Due to difficulty in accessing relevant tissues or biological fluids to measure drug levels, plasma drug concentrations are often collected with the hope that they will correlate with or be informative about exposures in compartments of interest, which is often found to be the case. Even between structurally similar compounds, however, penetration into these matrices may vary widely, and so exclusive use of plasma exposure in pharmacokinetic–pharmacodynamic (PK/PD) models may lead to suboptimal assessment of exposure–response relationships and inaccurate dose selection in certain circumstances. Differences in penetration between even structurally similar compounds into a given site may be due to a host of factors ranging from permeability to local metabolism to physicochemical properties to the role of drug transporters in efflux or uptake. Novel experimental and quantitative methods have been developed to assess drug levels at the site of action across therapies and disease areas. For central nervous system (CNS) diseases, the measurement of drug levels in the cerebrospinal fluid (CSF) using lumbar puncture and even directly in the brain using imaging or microdialysis has provided insight into drug penetration across the blood–brain barrier and helped characterize the compartmental pharmacology of such drugs. In oncology, linking information about drug penetration into solid tumors with radiologic outcomes provides a greater understanding of relevant exposure–response relationships. For HIV, characterization of drug distribution into the genital tract and the colorectum, sites of potential HIV exposure, has informed the development of preexposure prophylaxis. For antibacterials, lung penetration studies using microdialysis or invasive bronchiolar lavage have been instrumental in projecting drugs and doses with sufficient local exposure to be efficacious against the bacterial pathogens that cause pneumonia, while the assessment of PK in urine provides insight into the likelihood of successful treatment of urinary tract infections. As analytical and quantitative techniques have continued to evolve to allow for the assessment of drug levels at these various sites of action, so has our knowledge regarding the role of drug-metabolizing enzymes and transporters in the uptake and efflux of drugs in relevant compartments.1, 2 Our ability to assess the role a transporter may play in the distribution and elimination of a given drug has expanded with the development of increasingly complex in vitro systems and growing knowledge about how to translate such in vitro results to clinical impact. Together with quantitative approaches such as physiology-based pharmacokinetic (PBPK) modeling, we now have the tools at our disposal to predict drug disposition and how factors such as patient demographics, pharmacogenomics, or coadministration with other drugs affect it. There is a wealth of knowledge regarding the development and application of PBPK models that has been well described previously,3-5 both for small molecules and monoclonal antibodies.6, 7 With sufficient information about drug levels at the site of action, advances in population PK modeling can allow for the effect site to be treated as a separate compartment, thus bridging effect site PK to plasma PK. Once mathematical linkages between effect site and plasma PK are established, inferences about effect site PK can be made in subsequent experiments in which only plasma PK is collected, especially in cases where site-of-disease PK measurement is invasive or inconvenient. However, one must be careful to consider situations where nonlinearities may limit the ability to extrapolate plasma PK to tissue levels under different scenarios. Such limitations may arise due to saturation of drug transport pathways, nonlinear or concentration-dependent protein binding that differentially impacts fluids and tissues, or similarly nonlinear or concentration-dependent drug clearance. Additionally, often information regarding effect site PK is gathered in situations where limited sampling takes place. The dynamic changes in drug level in the plasma and effect site are often not synchronous, and thus sampling limitations may lead to an inadequate understanding of the relationship between plasma and effect site drug levels. Additionally, systems-level models such as quantitative systems pharmacology (QSP) and physiology-based pharmacodynamic (PBPD) models have progressed to enable linking the effect of drug levels at the site of action to the underlying causal biology.8, 9 Together, these quantitative approaches coupled with advances in sampling techniques allow for great advances in our ability to predict and describe the distribution of drugs in the body and the corresponding impact on disease modulation and treatment. However, it should be noted that each of these experimental and quantitative methodologies may come with significant effort, and thus the benefit of such approaches needs to be weighed against the utility of simpler methods. In many cases, fit-for-purpose translational models or the utilization of plasma PK may be perfectly suitable for the purpose at hand. Understanding PK at the site of action and demonstrating direct evidence of target engagement is not simply of scientific interest, but is also being increasingly recognized as key learning required to improve the yield of pharmaceutical research & development. Systematic reviews of drug discovery programs at large pharmaceutical companies have led to publication of translational risk frameworks, and obtaining adequate exposure at the intended site of action has been identified as a critical component of achieving downstream success in clinical trials.10, 11 Thus, the earlier one can demonstrate with confidence that a drug is present at the disease site, the higher the likelihood of observing the desired pharmacology for novel drug therapies. In this review, advances made in the estimation of drug exposures in several compartments of interest are discussed , as illustrated in Figure 1, including key biological considerations of each compartment and techniques utilized to gain insight into PK at the various sites of action, as described in Tables 1 and 2, respectively. The brain and the CSF are unique and difficult-to-access compartments. The blood–brain barrier (BBB) does an exquisite job of shielding the CNS from substances in the systemic circulation that are either nonessential or toxic. Tight junctions between capillary endothelial cells restrict paracellular diffusion, and most compounds that manage to enter (however briefly) are efficiently removed by efflux pumps.12 P-glycoprotein (P-gp, or MDR1) is the best-known transporter, but others, like breast cancer-resistant protein (BCRP), contribute to a highly redundant, efficient, and dynamic “waste” removal system.13 Drugs to treat CNS diseases such as brain cancer, meningitis, pain, psychiatric or neurodegenerative disorders must cross the BBB or blood–CSF (BCSF) barriers to be effective, and thus drug developers must outwit this highly evolved system. The ability of a drug to enter and remain in the CNS depends on several physicochemical properties—lipid solubility, ionization, molecular weight, the number of hydrogen bond donors, protein binding—and as predictive knowledge to assess these characteristics increases, scoring systems are being developed to assess a compound's drug-like properties for CNS penetration.14 Conversely, drugs intended for non-neurologic indications that readily enter the CNS may have off-target liabilities, such as efavirenz (HIV drug whose metabolite is directly neurotoxic and causes neurocognitive impairment) or cycloserine (anti-tuberculosis (TB) drug that is an NMDA agonist and provokes psychosis and symptoms of “irritability, hypocrisy and querulousness”).15-17 For treatment of a disease of the brain or meninges, drugs must adequately penetrate into the brain and CSF when given at doses that are clinically safe. Knowledge of drug concentrations at the site of (on-target or off-target) action may be helpful in guiding PK/PD assessments that, in turn, direct drug dosing decisions. However, the brain and CSF are neither readily nor repeatedly accessible compartments, and given differences between drug clearance from plasma vs. CSF as well as variability in penetration of drugs to the brain and CSF related to inflammation and breakdown of vascular integrity, the dynamics of drug concentrations in CSF cannot generally be extrapolated from plasma concentration data or accurately estimated from a single CSF-to-plasma paired-sample CSF collected lumbar puncture is often not a for drug concentrations in brain fluid are of the brain into and CSF is and There are also differences between BBB and with to the molecular of For while is the most transporter on the of the is at the the the other is at the the on the transporter it is that is critical at the in using CSF PK studies to assess CNS penetration be inaccurate drugs brain with and to CNS drug development has been with with CNS drugs an of to from to and only about an success as CNS drugs in are to be due to of more to assess both drug exposures in the brain more of drug activity at the site of disease in clinical development are microdialysis has as a of molecules in brain fluid and can be in models to an at drug penetration into the brain and one hope to be to characterize drug distribution relationships in an and simply CSF in and use relationships in the to predict brain it in not so In characteristics of the BBB and are across and and distribution for drugs may be similar in and however, CSF concentrations are generally higher CSF concentrations and higher variability may in CSF in studies is only collected in with a brain or CSF is collected in and brain drug distribution PBPK models developed using data CSF concentrations and often brain information about drug concentrations in brain and CSF microdialysis in however, can be to models and characterize such as and local tissue binding for drugs of different in the CNS only and and and information can be to the models physicochemical fluid transporter disease to improve predictive for estimating brain is increasingly being to metabolism clinically in with brain and techniques have been to measure drug concentrations of drugs in As more microdialysis data are differences in drug distribution in CNS compartments as well as of disease on CNS PK will increasingly be which will lead to predictive models of brain imaging has as a noninvasive for estimating CNS penetration of drugs. can also information about drug concentrations in of the brain to have a of a of interest or for psychiatric or in of brain disease in with disease or such as imaging can be clinical development of CNS to demonstrate in to assess exposure–response relationships by of target engagement or drug effect in to an to correlate drug exposure with brain and With with a disease of interest can be to the or of the drug of interest or a and activity in the of the brain can be molecules and monoclonal are both in the treatment of cancer, either more in tumors are by a complex and unique that of a and and vascular of vascular endothelial and such and by the tumors may and vascular permeability and In vitro studies that drug uptake and local concentrations are on the of the and the from the and the in and nonlinear local binding and metabolism can contribute to differences in PK and each of these has been in mathematical models that describe drug distribution in the and transporters of the protein have in by drug in in drug penetration between has been using molecular imaging with or monoclonal or small molecules have been developed against a of drug such as 1, and Such molecular imaging techniques allow for increasingly and noninvasive methods of therapies. can be a for drug concentrations the dosing in models and in accessible solid tumors in with that the for drugs is generally knowledge of site of action PK for drugs is for dose disposition at however, is often to predict direct as and and activity of local factors often between tumors and which is sampling of drug in the of the tissue of interest at which is more and informative the of tissue PK data at a single at the of in has been for small have been that also allow for the measurement of As by and the utility of microdialysis as an on achieving adequate of the of interest, which may be by a number of factors related to the the tissue of interest, and the of interest as Thus, each of these should be into when the utility of Additionally, of the microdialysis can the and impact drug distribution when is not to allow for sufficient for tissue to between can be in the for relevant site-of-disease As the treatment of to more or the ability to drug levels and not only in each but at each allow for a greater of understanding of the between the and drug and how of these factors to in a given has been to assess solid PK of for brain and breast others, in models or in with and is in drug The where drug distribution is by pharmacokinetic such as of drug transporters with to fluid limitations such as and has led to the development of modeling approaches that mathematical of fluid dynamics with PK models to and at the molecular and systemic As and approaches to the predictive of such methods will to for more and of drug penetration to enable approaches in of advances in the development of therapies for the treatment of HIV clinical studies have the utility of for the of HIV preexposure to the of the of and as to HIV in and results are by a number of from to but only when they are and the of to treatment across For both the and where results but also In several studies in where including the and of This has also led to the development of novel of such as and that are to and thus in to understand the potential for a to be for at the to gain information on PK measurement at the sites of potential exposure to HIV, the and genital tract is highly For an of drug levels of in the fluid of in the a between PK in the and the level of against HIV with with levels and with levels binding in the is to be with the of drug being these results are regarding the utility of is limited biological or for these and more needs to be to robust PK/PD in these as local drug concentrations in an are a of both drug disposition in that and of PK. assessment of the exposure–response of or other HIV is by the that these are by so concentrations of the drug are of most In the of to the whose levels to be when linkages between drug exposure and The PK of drugs in tissues is highly due both to patient and to the of drug metabolism and transport at these sites to thus plasma to be of limited as a for target site However, the measurement of tissue PK in the genital tract can invasive which the of data that can be and so is a greater on or population PK modeling to and the of fluids in studies has provided information about local PK the for invasive the of fluid direct provides insight into drug concentrations in the genital while the of fluid similarly provides insight into concentrations in the The between drug levels in these fluids and the tissues in one to use of local fluid as a for tissue fluid levels with corresponding tissue levels, they may be as a for the measurement of effect site PK. However, is limited information regarding protein binding in these tissues, which may limit of such data in the of related plasma concentration PK of drugs for HIV may that are more to benefit from For exposures are higher in while levels are similar to cause one to that at risk for HIV may be more to differences in have with are at risk to HIV and results of the in even with while the in in which in understanding the PK/PD relationship for HIV using advances in and clinical models have been Our understanding of differences in drug metabolism and transport increasingly to predict and understand differences in penetration into and The of enzymes in the and tissues has been and a this a the activity of and enzymes dosing of and as given the interest in these compounds for use as for HIV activity present in both the and and activity present in the Additionally, the found that the metabolite for in the and fluids different in the plasma or that levels of and enzymes in tissues may from in the growing body of knowledge regarding the activity of drug transporters in the genital tract and is also a greater understanding of PK at the site of action. of efflux and uptake transporters in these tissues of interest provides an of the body of to from this that drug transport can play a role in the tissue distribution of drugs. predictive data from drugs that the penetration to be and that this on or not the drug a of drug efflux transporters and Additionally, and have been found to be the genital while and are in tissue at levels higher or to in the The activity of these transporters is by factors such as disease and which may for the large variability in studies that have tissue PK of for HIV and in turn, that an understanding of drug transport is to predict tissue from the clinical in HIV with advances in our understanding of drug metabolism and that may be the advances in clinical and quantitative to our ability to predict the for the population to HIV is an and now an The of bacterial and has and for the development of therapies. In advances in and have to understand at a more level the to the of This in turn, for a more understanding of dose and of which therapies are most to be not only at the but also robust against can be by both and is most by the while and are the most causes of or can be a when with pneumonia, and dosing must be to the of methods have been to drug concentrations of in the including lavage fluid and There are various and of each of the methods that must be when an For pathogens may in the or For the is the site of action and is a where protein binding a limited role and the drug in the or For are as the site of interest. Thus, knowledge of the of the under may impact the of measurement Additionally, approaches such as may differences in the penetration of drug vs. or between different such as the measurement of drug levels in may either due to to or into the to the lung site of action to In lung penetration studies to the development of drug have to rely on lavage techniques for the measurement of which is to be the most relevant site for and The of the has been with the by the for for on the use of PK and in the development of antibacterials, where drug levels in the are as data to in of an of a drug for The and use of data for is described well in a review in which of including clinical studies in a number of is The review a number of key to consider in the of including the dosing the PK sampling and of PK the in of the the analytical techniques utilized to measure drug levels, and the subsequent data of which are to for drugs. levels in the to in plasma can vary widely, ranging from levels in plasma to that a of The of to plasma concentration can be by many drug including plasma protein the role of transporters in drug uptake and and the physicochemical properties of the Thus, it is to drug levels in the to drug levels in the plasma to gain a more robust understanding of lung penetration and a novel that to predict penetration on physicochemical properties using a quantitative activity relationship As such approaches to be and drug developers may gain more confidence in using to drug that are most to be in pneumonia, and invasive PK assessments can be or is by including with and that and into the that one on with with a of these to at one drug of a be effective, drugs in treatment must penetrate into the sites of and of must be present at the site of disease in adequate concentrations to drugs from of and should have activity against imaging is now being to characterize distribution of drugs into studies that activity of drugs to correlate with drug distribution into In clinical of lung disease is a of treatment for drugs but not and which may be in by differences in the ability of these drugs to penetrate into and in the of these The lung is also a key site of interest in understanding and the dosing of such as for the treatment of or disease In to a predictive to describe both the pharmacokinetics in the and the subsequent pharmacodynamic developed an experimental that of an coupled with a mathematical to describe the drug and of in both and This provides a novel that can be expanded to drug penetration and subsequent for other The PK of in the urine is of interest, for the treatment of urinary tract PK is to measure and of urine the of a dosing and similar methods for measurement as for the assessment of plasma PK. may various of metabolism and thus it is to the that a drug in the urine when the treatment of For is for the treatment of and only a single plasma concentrations are in 2 by elimination from the However, urinary concentrations are and remain for for In several such as and cannot be in the treatment of they are not and levels in the For only about of the dose of is in the drug tissue distribution and in with drug-metabolizing are for drug a PK of transporters in and plasma concentrations of many which may correlate with drug concentrations at the site of However, transporters may as a barrier for certain such as brain and the access of drugs to Conversely, drugs may be more in certain to it is to understand the role of transporters as of drug concentrations at the site of action. As described the most of drug the level of the efflux is in of this transporter the access of drugs to efflux transporters may also be of level of drugs. In vitro and clinical studies that transporter 1 is a key of the levels of and in with The level of drugs may also be by of both and efflux For is a of both uptake transporter and efflux transporter the level of in is on of both are also for drugs that are by For the of to is a of the concentration of a in the have a higher concentration of in cells when to this may the levels of drugs for in compartments such as For transporters the it from site of action in The role of transporters in the in is but it is that transporters play critical in the or of drugs with The transporters are for removal of from the of on of the the transporter the level of in the to to and in imaging an to transporters as of drug levels at the site of action. For that the transporter the transporter or allow to measure the of these transporters by various drugs in with In as a is by and protein 1 in the and which may information about the of the and of vs. key of understanding the likelihood of a drug to desired target modulation is the of drug exposure at the site of action. However, such an understanding can be by of and biological exposure at the site of action may not be in with levels, the utility of sampling as a such as in the cases where transport or is differences in protein binding in various fluids or tissues may and understanding of and should be when such experimental and quantitative that a on drug levels is In this review, advances made in estimating drug exposures in several compartments of interest are including key biological considerations of each compartment, as in 1, and techniques utilized to gain insight into PK at the various sites of action, as described in considerations regarding measurement of drug levels in the the tract and and in the urine as illustrated in Figure the by the role of drug transporters is As analytical techniques and the underlying knowledge of the to quantitative systems models can such knowledge and allow for insight to be our ability to predict and describe the distribution of drugs in the body and the corresponding impact on disease modulation and treatment. In cases where the efforts with such methods are and on fit-for-purpose approaches or the measurement of plasma PK is such approaches a significant in our understanding of drug The for is an of &