Mark A. Lovich

Arterial Paclitaxel Distribution and Deposition

Successful implementation of local arterial drug delivery requires transmural distribution of drug. The physicochemical properties of the applied compound, which govern its transport and tissue binding, become as important as the mode of delivery. Hydrophilic compounds distribute freely but are cleared rapidly. Hydrophobic drugs, insoluble in aqueous solutions, bind to fixed tissue elements, potentially prolonging tissue residence and biological effect. Paclitaxel is such a hydrophobic compound, with tremendous therapeutic potential against proliferative vascular disease. We hypothesized that the recent favorable preclinical data with this compound may derive in part from preferential tissue binding as a result of unique physicochemical properties. The arterial transport of paclitaxel was quantified through application ex vivo and measurement of the subsequent transmural distribution. Arterial paclitaxel deposition at equilibrium varied across the arterial wall and was everywhere greater in concentration than in the applied drug source. Permeation into the wall increased with time, from 15 minutes to 4 hours, and varied with the origin of delivery. In contrast to hydrophilic compounds, the concentration in tissue exceeds the applied concentration and the rate of transport was markedly slower. Furthermore, endovascular and perivascular paclitaxel application led to markedly differential deposition across the blood vessel wall. These data suggest that paclitaxel interacts with arterial tissue elements as it moves under the forces of diffusion and convection and can establish substantial partitioning and spatial gradients across the tissue. The complexity of paclitaxel pharmacokinetics requires in-depth investigation if this drug is to reach its full clinical potential in proliferative vascular diseases.

Mechanisms of Transmural Heparin Transport in the Rat Abdominal Aorta After Local Vascular Delivery

Local vascular drug delivery systems provide elevated concentrations in target arterial tissues, while minimizing systemic side effects. Drug can now be released to isolated arterial segments from the endovascular or perivascular aspects of the blood vessel, yet the forces that determine drug distribution and deposition for these different modes of delivery have not been rigorously investigated. This study examines mechanisms of transmural transport of a model vasoactive drug, heparin, and compares estimates of the distribution after administration from either aspect of the artery. We showed that (1) heparin traversed the arterial wall rapidly; (2) diffusion far outweighed convection in the control of transmural heparin transport in the normal artery, but after endothelial injury, convective forces rose to one quarter the magnitude of diffusive forces; (3) the endothelium posed a minimal diffusive barrier to heparin; and (4) the diffusive barrier imposed by the adventitia depended on its thickness. These findings strongly suggest that vasoregulatory compounds can be administered to target tissue by either perivascular or endovascular means with equal efficacy, because the forces governing transport of heparin from either aspect of the blood vessel wall are not significantly different. Furthermore, the differences in arterial transport properties between heparin and other macromolecules suggest that distribution and the optimal aspect of delivery will depend just as much on the physicochemical properties of the drug as the state of the blood vessel wall.

The Impact of Carrier Flow Rate and Infusion Set Dead- Volume on the Dynamics of Intravenous Drug Delivery

The dynamics of IV drug delivery resulting from drug infusions connected to main-line crystalloid carriers can be complex and depend on infusion set dead-volume, drug flow rate, and carrier flow rate. While the concept of dead-volume is intuitive, a lack of appreciation of the interaction with the carrier and drug flow rates can lead to unintended clinical effects resulting from large variations in the delivery rate of potent drugs. We derived mathematical models to quantify these interactions. Experimental simulation with methylene blue infusions tested these predictions. The models predict a lag in response time to changes in carrier or drug flow, which is proportional to the dead-volume and inversely related to the total flow rate. Increasing the carrier rate provides an acute drug bolus. Temporary reduction or cessation of carrier flow decreases the rate of drug delivery, potentially for prolonged periods. Furthermore, a drug bolus results from restoration of the carrier flow. The method of connecting an infusion to a carrier and the use history affects the dynamics of drug delivery. Thus, although complex, the impact of infusion set architecture and changes in carrier and drug flow rates are predictable. These quantitative studies may help optimize the safe use of IV drug infusion systems.

The delivery of drugs to patients by continuous intravenous infusion: modeling predicts potential dose fluctuations depending on flow rates and infusion system dead volume.

IV drug infusion has the potential for dosing errors, which arise from complex interactions between carrier flows and the infusion set dead volume. We computed the steady-state mass of drug stored in the infusion set dead volume, using phenylephrine as a model compound. The mass of drug in the dead volume increases with stock drug concentration and desired dose but decreases with carrier flow rate. We also modeled the dynamic perturbations in drug delivery when a carrier is abruptly stopped. Rapid initial carrier flow rates lead to greater depression in drug delivery rate after carrier flow ceases. Rapid drug infusion rates lead to faster restoration of desired drug delivery. Finally, the time to reach a new steady-state after a change in drug delivery or carrier rate was computed. This time is longest for large stock-drug concentrations, larger dead volumes, and slower final carrier rates. These computations illustrate that (a) the dead volume may contain a large mass of drug available for inadvertent bolus, (b) cessation of carrier flow can profoundly reduce drug delivery, and (c) after a change in carrier flow or drug dosing, a significant lag is possible before drug delivery achieves steady state. Although computed for phenylephrine, the concepts are generic and valid for any drug administered by IV infusion.

Arterial heparin deposition: role of diffusion, convection, and extravascular space.

Transvascular transport has been studied with atherogenic, tracer, and inert compounds such as low-density lipoprotein, horseradish peroxidase, and albumin, respectively. Few studies used vasoactive compounds, and virtually all studies examined entry from the lumen and not from the perivascular space. We compared several mechanisms that govern arterial heparin deposition after administration to the perivascular and endovascular aspects of the calf carotid artery in vitro and the rabbit iliac artery in vivo. In the absence of transmural hydrostatic pressure gradients, heparin deposition following endovascular administration was unaffected by deendothelialization and was indistinguishable from perivascular delivery. Deposition in the former was enhanced by the addition of a pressure gradient and to a greater extent in denuded arteries, indicating that convection influences transport but is dampened by the endothelium. Neither the endothelium nor the adventitia pose significant resistances to heparin. Deposition in vivo was greater following endovascular hydrogel release than perivascular application from similar devices to native or denuded arteries. The loss of drug to extra-arterial microvessels exceeded the loss of drug to the lumen flow. These findings are essential for describing vascular pharmacokinetics and for implementing local pharmacotherapies.Transvascular transport has been studied with atherogenic, tracer, and inert compounds such as low-density lipoprotein, horseradish peroxidase, and albumin, respectively. Few studies used vasoactive compounds, and virtually all studies examined entry from the lumen and not from the perivascular space. We compared several mechanisms that govern arterial heparin deposition after administration to the perivascular and endovascular aspects of the calf carotid artery in vitro and the rabbit iliac artery in vivo. In the absence of transmural hydrostatic pressure gradients, heparin deposition following endovascular administration was unaffected by deendothelialization and was indistinguishable from perivascular delivery. Deposition in the former was enhanced by the addition of a pressure gradient and to a greater extent in denuded arteries, indicating that convection influences transport but is dampened by the endothelium. Neither the endothelium nor the adventitia pose significant resistances to heparin. Deposition in vivo was greater following endovascular hydrogel release than perivascular application from similar devices to native or denuded arteries. The loss of drug to extra-arterial microvessels exceeded the loss of drug to the lumen flow. These findings are essential for describing vascular pharmacokinetics and for implementing local pharmacotherapies.

central venous catheter infusions: A laboratory model shows large differences in drug delivery dynamics related to catheter dead volume *

Objective: Central venous catheters (CVCs) are conduits for drug infusions. Dead volumes of different CVC lumens vary considerably. This study quantitatively evaluated drug delivery dynamics of CVCs in a laboratory model of continuous drug infusion. Design: CVCs studied included a triple‐lumen catheter (16‐gauge and 18‐gauge lumens), the proximal infusion port of a pulmonary artery catheter, and a 9‐Fr introducer sheath, with and without a pulmonary artery catheter in the lumen. One syringe pump infused a carrier. A second pump infused the model drug methylene blue (3 mL/hr), joining the carrier immediately upstream of the CVC. Samples were collected every minute for quantitative analysis. Setting: Laboratory model. Subjects: None. Interventions: At low fixed flow rates, experiments characterized drug delivery kinetics of different CVCs. Data collection then assessed effects of increased carrier flow. Measurement and Main Results: The time to steady‐state delivery after initiation of methylene blue infusion differed between CVCs. At a carrier flow of 10 mL/hr, the fastest achievement of steady‐state delivery was with the 18‐gauge lumen of a triple‐lumen catheter. The 9‐Fr introducer had the slowest time to achieve steady‐state delivery. Other CVCs had intermediate kinetics. Reducing drug delivery from steady state to zero after cessation of methylene blue infusion was fastest with the 18‐gauge lumen and slowest with the 9‐Fr introducer. Increasing carrier flow rates from 10 to 60 mL/hr hastened the time to target for initiation and cessation of methylene blue delivery. Conclusions: Experiments demonstrate large differences between CVCs in the dynamics for delivery of model drug methylene blue. Achieving targeted steady‐state delivery, and termination of a planned continuous drug infusion, may be far slower than typically appreciated. Delivery kinetics depend on the dead volume and the rate of carrier flow. Safe and effective management of continuous drug infusions depends on understanding the dynamics of the delivery system.

Drug Clearance and Arterial Uptake After Local Perivascular Delivery to the Rat Carotid Artery

OBJECTIVES We attempted to characterize how drug released into the perivascular space enters the arterial wall and how it is cleared from the local environment. BACKGROUND Drug released into the perivascular space can enter the artery either from the adventitial aspect or from the lumen after absorption by the extraarterial capillaries and mixing within the systemic circulation. Some investigators suggest that this latter mechanism dominates, and they question whether local drug release is synonymous with local deposition. METHODS We investigated both the pathways by which adventitially released drug is cleared from the perivascular space and those by which drug enters the blood vessel wall. Inulin was used to follow drug release from implanted devices and subsequent entry to the circulation, because of its first-pass urinary excretion. Heparin was used to follow arterial deposition because of its vasoactivity and tissue-binding properties. The different potential pathways of drug entry and egress were systematically removed and the effects on metabolism and deposition determined. RESULTS Ligature occlusion of the artery did not decrease inulin excretion or heparin deposition. Extravascular wraps designed to shield the device from extramural capillaries reduced inulin excretion rates 10-fold but did not alter heparin deposition into the vessel wall. The deposition of drug after perivascular delivery was 500 times higher than after intraperitoneal administration. CONCLUSIONS Although almost all the drug released into the perivascular space is cleared through the extravascular capillaries, virtually all the deposited drug diffuses directly from the perivascular space, and little arrives from the endovascular aspect. These data support the view that local drug release leads directly to increased local drug concentration.

An In Vitro Analysis of Central Venous Drug Delivery by Continuous Infusion: The Effect of Manifold Design and Port Selection

BACKGROUND: Central venous catheters are used extensively in anesthesia and critical care. Multiport manifolds allow for simultaneous administration of multiple medication infusions into a common central venous catheter lumen. The structures of such manifolds vary considerably. In this study, we quantitatively compared, in a laboratory model of continuous drug infusion, the drug delivery dynamics of a traditional stopcock manifold and a microinfusion manifold constructed to minimize dead volume. METHODS: A syringe pump infused a saline carrier solution at a low flow rate frequently used in an intensive care unit (10 mL/h) through a multiport manifold connected to the 16-gauge lumen of a standard 16-cm triple-lumen catheter. The model drug methylene blue (3 mL/h) joined the carrier flow at the first, second, or fourth stopcock of a traditional manifold or 1 of 2 positions in a microinfusion manifold, a new device designed to minimize dead volume. Effluent samples were collected every minute for quantitative spectrophotometric analysis of delivery onset and offset. RESULTS: Onset and offset times differed significantly among individual ports of the traditional 4-stopcock manifold. There was also a significant difference between the 2 ports of the microinfusion manifold, but this was less pronounced. Both ports of the microinfusion manifold yielded delivery dynamics that were similar to the most downstream port of the 4-stopcock manifold. There was good correlation between dynamic data and dead volume for each of the manifolds. CONCLUSIONS: Using a traditional stopcock manifold, port selection significantly affects drug delivery dynamics for continuous infusions. The findings provide quantitative support for the concept that the most critical infusion should join the system at the manifold port closest to the patient. Port selection was less important for the microinfusion manifold and dynamics were faster compared with the second and fourth ports of the stopcock manifold. The smaller dead volumes of the microinfusion manifold minimize unwanted delays in drug delivery onset and offset allowing more precise control over drug delivery by continuous infusion.

Quantitative analysis of continuous intravenous infusions in pediatric anesthesia: safety implications of dead volume, flow rates, and fluid delivery.

Objective:  Quantitative characterization of continuous pediatric drug infusions.

Correlation of transarterial transport of various dextrans with their physicochemical properties.

Local vascular drug delivery provides elevated concentrations of drug in the target tissue while minimizing systemic side effects. To better characterize local pharmacokinetics we examined the arterial transport of locally applied dextran and dextran derivatives in vivo. Using a two-compartment pharmacokinetic model to correct the measured transmural flux of these compounds for systemic redistribution and elimination as delivered from a photopolymerizable hydrogel surrounding rat carotid arteries, we found that the diffusivities and the transendothelial permeabilities were strongly dependent on molecular weight and charge. For neutral dextrans, the effective diffusive resistance in the media increased with molecular weight approximately 4.1-fold between the molecular weights of 10 and 282 kDa. Similarly, endothelial resistance increased 28-fold over the same molecular weight range. The effective medial diffusive resistance was unaffected by cationic charge as such molecules moved identically to neutral compounds, but increased approximately 40% when dextrans were negatively charged. Transendothelial resistance was 20-fold lower for the cationic dextrans, and 11-fold higher for the anionic dextrans, when both were compared to neutral counterparts. These results suggest that, while low molecular weight drugs will rapidly traverse the arterial wall with the endothelium posing a minimal barrier, the reverse is true for high molecular weight agents. With these data, the deposition and distribution of locally released vasotherapeutic compounds might be predicted based upon chemical properties, such as molecular weight and charge.