Pharmacokinetic considerations of regional administration and drug targeting: influence of site of input in target tissue and flux of binding protein

When to consider intra-target microdosing: physiologically based pharmacokinetic modeling approach to quantitatively identify key factors for observing target engagement

Intra-Target Microdosing (ITM), integral to Phase 0 clinical studies, offers a novel approach in drug development, effectively bridging the gap between preclinical and clinical phases. This methodology is especially relevant in streamlining early drug development stages. Our research utilized a Physiologically Based Pharmacokinetic (PBPK) model and Monte Carlo simulations to examine factors influencing the effectiveness of ITM in achieving target engagement. The study revealed that ITM is capable of engaging targets at levels akin to systemically administered therapeutic doses for specific compounds. However, we also observed a notable decrease in the probability of success when the predicted therapeutic dose exceeds 10 mg. Additionally, our findings identified several critical factors affecting the success of ITM. These encompass both lower dissociation constants, higher systemic clearance and an optimum abundance of receptors in the target organ. Target tissues characterized by relatively low blood flow rates and high drug clearance capacities were deemed more conducive to successful ITM. These insights emphasize the necessity of taking into account each drug's unique pharmacokinetic and pharmacodynamic properties, along with the physiological characteristics of the target tissue, in determining the suitability of ITM.

Drug structure-transport relationships

Malcolm Rowland has greatly facilitated an understanding of drug structure–pharmacokinetic relationships using a physiological perspective. His view points, covering a wide range of activities, have impacted on my own work and on my appreciation and understanding of our science. This overview summarises some of our parallel activities, beginning with Malcolm’s work on the pH control of amphetamine excretion, his work on the disposition of aspirin and on the application of clearance concepts in describing the disposition of lidocaine. Malcolm also spent a considerable amount of time developing principles that define solute structure and transport/pharmacokinetic relationships using in situ organ studies, which he then extended to involve the whole body. Together, we developed a physiological approach to studying hepatic clearance, introducing the convection–dispersion model in which there was a spread in blood transit times through the liver accompanied by permeation into hepatocytes and removal by metabolism or excretion into the bile. With a range of colleagues, we then further developed the model and applied it to various organs in the body. One of Malcolm’s special interests was in being able to apply this knowledge, together with an understanding of physiological differences in scaling up pharmacokinetics from animals to man. The description of his many other activities, such as the development of clearance concepts, application of pharmacokinetics to the clinical situation and using pharmacokinetics to develop new compounds and delivery systems, has been left to others.

Physiologically based toxicokinetic models and their application in human exposure and internal dose assessment

Human populations may exhibit large interindividual variation in toxicokinetic response to chemical exposures. Rapid developments in dosimetry research have brought medicine and public health closer to understanding the biological basis of this heterogeneity. The toxicokinetic behavior of chemicals is, in part, controlled by the properties of the epithelium surrounding organs, some of which are effective barriers to penetration into the systemic circulation. Physiologically based toxicokinetic (PBTK) models have been developed and used to simulate the mechanism of uptake into the systemic circulation, to extrapolate between doses and exposure routes, and to estimate internal dosimetry and sources of heterogeneity in animals and humans. Recent improvements to PBTK models include descriptions of active transport across biological membranes, carrier-mediated clearance, and fractal kinetics. The expanding area of toxicogenetics has provided valuable insight for delineating toxicokinetic differences between individuals; genetic differences include inherited single nucleotide polymorphisms, copy number variants, and dynamic changes in the methylation pattern of imprinted genes. This chapter discusses the structure of PBTK models and how toxicogenetic information and newer biological descriptions have improved our understanding of variability in response to toxicant exposures.

Applying models of targeted drug delivery to gene delivery

Gene delivery requires targeted delivery systems. Exploratory simulations using models of targeted drug delivery helps one assess the worthiness of such systems, and helps quantify the expected therapeutic benefits of the systems. The drug targeting index (DTI), a ratio of availabilities, is a measure of pharmacokinetic benefit of the delivery device, based on a combination of a physiologically-based pharmacokinetic model and a single pharmacodynamic E<inf>max</inf>model. Pharmacodynamic outcomes are quantified by the degree of separation between the dose-response and dose-toxicity curves (SRT). Simulations are undertaken to investigate the potential linkage of DTI and SRT, a pharmacodynamic outcome. A significant positive linear relationship is found between the DTI and SRT. The relationship can be translated into a minimum pharmacokinetic requirement that can be used to guide making decisions regarding whether or not further pursue the development of a candidate gene-delivery device as a therapeutic agent.

Synovial fluid levels and serum pharmacokinetics in a large animal model following treatment with oral glucosamine at clinically relevant doses

OBJECTIVE

To examine the concentration of glucosamine in the synovial fluid and its pharmacokinetics in serum in a large animal model following dosing with glucosamine HCl at clinically relevant levels.

METHODS

Eight adult female horses were studied. After an overnight fast, glucosamine HCl (20 mg/kg of body weight) was administered by either nasogastric (NG) intubation or intravenous (IV) injection. Blood samples were collected before dosing and at 5, 15, 30, 60, 120, 180, 240, 360, 480, and 720 minutes after dosing. Synovial fluid samples were collected from the radiocarpal joints 48 hours before dosing and at 1 and 12 hours after dosing. Glucosamine was assayed by fluorophore-assisted carbohydrate electrophoresis.

RESULTS

The maximum concentration of glucosamine in serum reached approximately 300 muM ( approximately 50 microg/ml) following IV dosing and approximately 6 microM (approximately 1 microg/ml) following NG dosing. Synovial fluid concentrations reached 9-15 microM with IV dosing and 0.3-0.7 microM with NG dosing, and remained elevated (range 0.1-0.7 microM) in most animals even at 12 hours after dosing. Following NG dosing, the median serum maximal concentration of 6.1 microM (range 4.38-7.58) was attained between 30 minutes and 4 hours postdose. The mean apparent volume of distribution was 15.4 liters/kg, the mean bioavailability was 5.9%, and the mean elimination half-life was 2.82 hours.

CONCLUSION

Clinically relevant dosing of glucosamine HCl in this large monogastric animal model results in serum and synovial fluid concentrations that are at least 500-fold lower than those reported to modify chondrocyte anabolic and catabolic activities in tissue and cell culture experiments. We conclude that the apparent therapeutic benefit of dietary glucosamine on pain and joint space width in humans and animals may be secondary to its effects on nonarticular tissues, such as the intestinal lining, liver, or kidney, since these may be exposed to much high levels of glucosamine following ingestion.

Delivery of neurotrophic factors to the central nervous system: pharmacokinetic considerations

Neurotrophic factors are proteins with considerable potential in the treatment of central nervous system (CNS) diseases and traumatic injuries. However, a significant challenge to their clinical use is the difficulty associated with delivering these proteins to the CNS. Neurotrophic factors are hydrophilic, typically basic, monomeric or dimeric proteins, mostly in the size range of 5 to 30 kDa. Neurotrophic factors potently support the development, growth and survival of neurons, eliciting biological effects at concentrations in the nanomolar to femtomolar range. They are not orally bioavailable and the blood-brain and blood-cerebrospinal fluid barriers severely limit their ability to enter into and act on sites in the CNS following parenteral systemic routes of administration. Most neurotrophic factors have short in vivo half-lives and poor pharmacokinetic profiles. Their access to the CNS is restricted by rapid enzymatic inactivation, multiple clearance processes, potential immunogenicity and sequestration by binding proteins and other components of the blood and peripheral tissues. The development of targeted drug delivery strategies for neurotrophic factors will probably determine their clinical effectiveness for CNS conditions. Achieving significant CNS target site concentrations while limiting systemic exposure and distribution to peripheral sites of action will lessen unwanted pleiotropic effects and toxicity. Local introduction of neurotrophic factors into the CNS intraparenchymally by direct injection/infusion or by implantation of delivery vectors such as polymer matrices or genetically modified cells yields the highest degree of targeting, but is limited by diffusion restrictions and invasiveness. Delivery of neurotrophic factors into the cerebrospinal fluid (CSF) following intracerebroventricular or intrathecal administration is less invasive and allows access to a much wider area of the CNS through CSF circulation pathways. However, diffusional and cellular barriers to penetration into surrounding CNS tissue and significant clearance of CSF into the venous and lymphatic circulation are also limiting. Unconventional delivery strategies such as intranasal administration may offer some degree of CNS targeting with minimal invasiveness. This review presents a summary of the neurotrophic factors and their indications for CNS disorders, their physicochemical characteristics and the different approaches that have been attempted or suggested for their delivery to the CNS. Future directions for further research such as the potential for CNS disease treatment utilising combinations of neurotrophic factors, displacement strategies, small molecule mimetics, chimaeric molecules and gene therapy are also discussed.

Cellular microtransport processes: intercellular, intracellular, and aggregate behavior

Ionic and molecular transfer among cells occurs by a variety of transport processes operative at different length scales. Cell membrane permeability and electrical conductance derive from channel proteins producing pores at the molecular (ultrastructural) scale. Intracellular mobility involves the dynamics of motion through the complex ultrastructure of the cytoplasm. These phenomena unite in the larger-scale (microscopic) process of gross intercellular transfer. When such movement occurs among sufficiently many cells, it in turn begins to reflect their average collective (macroscopic) behavior as bulk tissue. This article surveys selected aspects of intercellular and intracellular transport, with emphasis on detailed mechanistic theory, experimental probes of cellular permeability, and systematic transcendence from small to large length scales.