Corresponding Author: Angela D. M. Kashuba, PharmD, 1094 Genetic Medicine Building, CB# 7361, 120 Mason Farm Road, UNC Eshelman School of Pharmacy, Division of Pharmacotherapy and Experimental Therapeutics, University of North Carolina at Chapel Hill, North Carolina, NC 27599, Tel (919) 966-9998 Fax (919) 962-0644, ude.cnu@abuhsaka
The publisher's final edited version of this article is available at Clin PharmacokinetDespite contributing significantly to the burden of global disease, the translation of new treatment strategies for diseases of the central nervous system (CNS) from animals to humans remains challenging, with a high attrition rate in the development of CNS drugs. The failure of clinical trials for CNS-therapies can be partially explained by factors related to pharmacokinetics/pharmacodynamics (PKPD) such as lack of efficacy or improper selection of initial dosage. A focused assessment is needed for CNS-acting drugs in first-in-human studies to identify the differences in PKPD from animal models as well as choose the appropriate dose. In this review, we summarize available literature from human studies on the pharmacokinetics and pharmacodynamics in brain tissue, cerebrospinal fluid and interstitial fluid for drugs used in the treatment of psychosis, Alzheimer’s disease and neuro-HIV and address critical questions in the field. We also explore newer methods to characterize pharmacokinetic/pharmacodynamic relationships that may lead to more efficient dose selection in CNS drug development.
Disorders of the brain contribute significantly to global disease burden. Psychiatric, neurological, developmental and substance abuse disorders affect more than 1 billion people worldwide.(1) As of 2010, these were the leading cause of years lived with disability (YLD) globally, accounting for approximately 30% of all YLDs.(2) However, CNS drug development is extremely challenging. Compared to non-CNS drug development, these programs have a lower clinical approval rate (6% versus 13%) and a longer time to market (12 years versus 6–7 years).(3–6) This has led to several companies withdrawing drug development programs in the neurosciences(7–9) signaling an uncertain future for novel research in CNS disorders.
Difficulty selecting initial drug dosage, untoward toxicities and lack of efficacy are cited as some driving forces behind the high attrition rate of CNS therapies.(10) A robust concentration-effect analysis can provide valuable, reproducible information regarding both the therapeutic as well as adverse effect drug profile over a wide range of doses and greatly aid the development of CNS-acting drugs. However, a report from 2007 indicated that there were very few sets of pharmacodynamic data generated from human studies over a wide range of doses or concentrations.(11) Although concentration-effect relationships are assessed in animals, animal models do not always accurately predict human disease, especially in case of CNS disorders.(12) Difference in blood-brain barrier (BBB) permeability, drug metabolizing enzymes and transporters can lead to differences in drug exposure in the human brain compared to animals(13) and only rarely can drug be sampled from the human brain for pharmacokinetic (PK) measures. Further, animal models may only mimic some mechanisms of human CNS disease or contain targets not seen in humans, challenging the translation of efficacy and/or toxicity of novel therapeutics. Therefore, to address these issues a focused pharmacokinetic/pharmacodynamic (PKPD) assessment is required in humans to identify differences from animal models and adjust dosing. This has been accomplished by employing alternate methodologies such as in-vitro systems, translational studies or in-silico modelling to supplement the understanding of pharmacology within the CNS.
This review is broadly divided into three parts. In section 3, existing methods to measure PK and PD in the brain tissue, cerebrospinal fluid (CSF) and interstitial fluid (ISF) are reviewed. While there is abundance of PKPD information from animal models in the CNS, less complete information is available from human studies. In sections 4–6, we examine clinical PKPD analyses at relevant target sites in the CNS for antipsychotics, anti-Alzheimer’s drugs and antiretrovirals and examine the utility of available information and the need for more research to answer critical questions in the field. In the absence of clinical results, available animal data are presented and cautiously interpreted for clinical relevance. Finally, new methods to improve CNS drug development are examined in section 7.
An extensive literature search was performed to identify research articles and conference abstracts published in Embase (including articles in the MEDLINE® database) using terms for drugs used to treat disorders of the brain and CNS, combined with terms for PK or PD and terms for the brain and CNS. A full search strategy is provided in the supplementary material. These searches were augmented by targeted searches in PubMed, Google Scholar, and Google Books, which combined terms from the full search strategy, plus additional terms for PK or PD measures or factors affecting these measures. The bibliographies of relevant review articles were also hand searched for additional relevant studies.
Drug distribution into the CNS has been characterized by several methods: measuring drug uptake into cultured brain cells (in-vitro), or measuring drug concentration in the brain tissue (ex-vivo) or CSF or ISF (in-vivo).
In-vitro models of the BBB are used as a first line-approach for determining the extent to which investigational agents cross into the brain(14). There are several validated models of the BBB from multiple species(15) and while no ideal cell line exists, the human cell line most widely used and well characterized is the human immortalized endothelial cell line hCMEC/D3. hCMEC/D3 experiments can quantify drug permeability, identify relevant drug-efflux transporter interactions, rapidly screen drug candidates for CNS activity and carry out initial PK studies. However, these models are a static measure of drug PK. For anti-infectives in particular, these models do not account for time-dependent killing and may be less clinically relevant. In-vitro systems also do not fully replicate all in-vivo features of the BBB. For example hCMEC/D3 is more “leaky” than the BBB, and can express lower levels of BBB-specific enzymes and drug transporters(15). Therefore, in-vitro systems may have to undergo modification such as co-culture with other brain cells to replicate tight junctions of BBB.(16) Newer microfluidic technologies such as BBB-on-a-chip or neurovascular-unit-on-a-chip(17) hold promise to mimic the dynamic in-vivo environment.
There are several ex-vivo approaches to measuring drug concentrations in brain tissue either after surgical resection or necropsy. Most PK information comes from brain tissue homogenates using liquid chromatography-mass spectrometry (LC-MS) analysis. These measurements are then used to calculate ISF and intra-cellular fluid (ICF) concentrations(18). Though commonly used, these methods do not provide information about drug localization. Mass spectrometry (MS) imaging has emerged as a method to quantify drug molecules by MS and spatially visualize drug distribution in tissue slices.(19) The advantage of MS imaging is that it can capture drug distribution patterns within different regions of a tissue(20). For example, using Matrix Assisted Laser Desorption Ionization (MALDI) imaging MS, the anti-tubercular drug pretomanid was found to localize predominantly in the corpus callosum of Sprague Dawley rats(21). By using serial sections collected at different time points, it was shown that pretomanid distributed into the corpus callosum 1–2 hours after an intraperitoneal dose of 20mg/kg, and diffused into other parts of the brain at later time points. With advances in imaging technology, this technique may be used to image intracellular drug concentrations and can be coupled with PD targets through immunohistochemistry (IHC) or in-situ hybridization in contiguous slices. While this has not yet been demonstrated for brain cells, Aikawa et al. used hematoxylin and eosin (H&E) along with IHC staining for CD31 and multidrug resistance transporter 1 (MDR1) to show the colocalization of anti-cancer drug alectinib with blood vessels in murine brains(22). A drawback of ex-vivo imaging is that it is a static measurement, and a composite of multiple images from different animals is required to gain information across a dosing interval.
In-vivo imaging techniques, such as Positron Emission Tomography (PET), can provide longitudinal information on drug disposition. PET is a non-invasive imaging technique that relies on the detection of radio-labelled ligands over time. It has been used to measure absolute spatial concentration of drug and determine PK parameters as well as target occupancy of several CNS-acting drugs. While a detailed discussion of PET is beyond the scope of this review, the reader is directed to a 2013 review(23) for a detailed summary on estimating PK parameters using PET studies. Despite the spatial advantages and applicability to human studies, PET scans are expensive, generally limited to fewer patients because of the use of radioactivity, and may not distinguish between parent compound and metabolites.
Other in-vivo drug estimation methods measure drug penetration into fluid compartments of the CNS. Microdialysis involves inserting a dialysis probe into the cerebral region of the brain to measure the protein-unbound concentration in the ISF. This technique is regularly used in animal models for continuous monitoring of drug concentration, but is only applicable during intra-operative procedures in humans.(24) Further, this procedure might not be suitable to measure the concentration of highly lipophilic or protein bound drugs as there can be a high degree of non-specific binding to the microdialysis probe and poor recovery of drug from the fluid.(24,25) Additionally, intracellular active metabolites are not captured using this technique.
The most common approach to generating PK data is drug sampling in CSF. This is done by lumbar puncture for a single sample and spinal catheterization in the subarachnoidal space for continuous sampling. While less invasive than microdialysis, lumbar punctures are painful and not without medical risks, and are not routinely performed. Also, concentrations measured by lumbar puncture can differ based on the location and time of measurement(13). For example, using a mathematical model, phenytoin was predicted to reach 300% greater concentration in cranial CSF than spinal CSF(26). Generally, unbound CSF concentrations are used as surrogates for unbound brain tissue concentrations in animal models(27) based on the free drug hypothesis which stipulates that protein-unbound drug passively moves from the plasma through the BBB and blood-CSF barrier (BCSFB) into the brain and CSF(28). However, this generalization holds true for certain drugs(29,30) with two significant exceptions: i) Drugs that use membrane transporters for influx and efflux (Eg. antidepressants, antiretrovirals [ARVs]) and ii) Drugs with low permeability to cross through the BBB where CSF bulk flow exceeds passive diffusion of the compound into CSF.(31) For substrates of efflux membrane transporters such as P-gp, CSF concentrations tend to overestimate ISF concentrations.(32) While the exact reason for this observation remains unknown, some hypotheses include subapical or apical localization of P-gp on the choroid plexus that results in drug transfer and accumulation into the CSF,(33) or non-functionality of P-gp at the BCSFB.(34) Since the CSF is recycled at a faster rate than ISF, the CSF acts as a “sink” to clear drug.(31) For high permeability compounds, this effect is negligible but for low-permeability compounds, CSF concentrations underestimate the brain or ISF concentrations (Eg. morphine 6-glucuronide). Therefore, the unbound concentration in the brain may differ from the CSF concentration and confound target site assumptions.
In case of in-vivo measurements made at a single time point, the concentration of drug in brain or CSF may be normalized to a simultaneously-collected plasma concentration. While this is a common means of estimating the extent of drug uptake into the CNS, and allows for comparisons of uptake between drugs, the rates of entry and elimination of the drug in plasma, CSF and brain compartments differ.(35) For example, the CSF:plasma concentration ratio for ciprofloxacin increases by as much as 1400% over 24 hours.(35) One approach to avoid this confounding is to use sparse serial sampling in a group of animals or humans to characterize the drug’s full PK profile in the CSF and plasma and calculate the ratio of drug exposure in the 2 compartments by measuring the area under the concentration-time curve. This approach has been performed for several anti-infective drugs(36) during ventricle catheterization when CNS infections need to be monitored(37) or excess CSF fluid needed to be drained.(38,39) Due to difficulties in obtaining multiple CSF samples from patients, population PK modeling has been used with sparse CSF and plasma sampling in order to obtain exposure profiles of various drugs such as abacavir.(40)
When considering the site of action, it is important to distinguish between extracellular and intracellular CNS drug concentrations. For drugs that act on receptors on neuronal cell membranes such as anti-epileptic drugs (AEDs) and anti-Alzheimer’s drugs, it is preferable to measure drug concentration in the ISF where the PD effect is exerted. Extracellular acting drugs have been measured in brain tissue homogenates, but this approach may be misleading. For AEDs and other basic drugs (pKa >7) where brain volume of distribution is greater than brain water volume (0.8 mL/g), ISF concentrations are over-estimated by brain tissue homogenate due to non-specific binding in brain tissue.(41,42) For anti-infective and anti-cancer drugs which act on intracellular targets, the unbound intracellular drug concentration is the most appropriate PK measure linked with activity. Friden and colleagues demonstrated a method to indirectly estimate unbound intracellular drug concentration. Briefly, in-vitro volume of distribution of unbound drug in brain (Vu,brain) is measured in brain slices from drug-naïve animals incubated in drug containing buffer (brain slice method(43)) and fraction of unbound drug in the brain (fu,brain) is measured by adding drug to brain homogenates from drug-naïve animals.(18) The ratio of intracellular to extracellular unbound drug concentration (Kp,uu,cell) is given by equation 1.
Kp,uu,cell = Vu,brain ∗ fu,brainUsing this method, intracellular drug concentrations of gabapentin, oxycodone, morphine and codeine were found to be greater than extracellular concentrations.(18)