Mechanisms of transmural heparin transport in the rat abdominal aorta after local vascular delivery

Mechanisms Underlying Drug Delivery to Peripheral Arteries

Delivery of drugs onto arterial targets via endovascular devices commands several principles: dissolution, diffusion, convection, drug binding, barriers to absorption, and interaction between the drug, delivery vehicle, and accepting arterial wall. The understanding of drug delivery in the coronary vasculature is vast; there is ongoing work needed in the peripheral arteries. There are differences that account for some failures of application of coronary technology into the peripheral vascular space. Breakthroughs in peripheral vascular interventional techniques building on current technologies require investigators willing to acknowledge the similarities and differences between these different vascular territories, while developing technologies adapted for peripheral arteries.

Perivascular medical devices and drug delivery systems: Making the right choices

Perivascular medical devices and perivascular drug delivery systems are conceived for local application around a blood vessel during open vascular surgery. These systems provide mechanical support and/or pharmacological activity for the prevention of intimal hyperplasia following vessel injury. Despite abundant reports in the literature and numerous clinical trials, no efficient perivascular treatment is available. In this review, the existing perivascular medical devices and perivascular drug delivery systems, such as polymeric gels, meshes, sheaths, wraps, matrices, and metal meshes, are jointly evaluated. The key criteria for the design of an ideal perivascular system are identified. Perivascular treatments should have mechanical specifications that ensure system localization, prolonged retention and adequate vascular constriction. From the data gathered, it appears that a drug is necessary to increase the efficacy of these systems. As such, the release kinetics of pharmacological agents should match the development of the pathology. A successful perivascular system must combine these optimized pharmacological and mechanical properties to be efficient.

Towards the development of an in vitro model of atherosclerotic peripheral vessels for evaluating drug-coated endovascular technologies

Here, we review the in vitro models used to evaluate drug-coated endovascular technologies. The models are assessed in the context of representing the drug transport/uptake and mechanical properties of atherosclerotic peripheral vessels. Studies to date have incorporated a vessel-simulating hydrogel compartment to examine drug elution from endovascular devices. However, comparisons between in vitro models and atherosclerotic tissue are difficult because ex vivo data are limited in their applicability to diseased peripheral vessels. Furthermore, appropriate ex vivo mechanical properties are not incorporated into these models. Therefore, there is a need to characterise the drug transport/uptake properties of appropriate atherosclerotic tissue and incorporate existing ex vivo mechanical data into current in vitro models to more accurately represent drug behaviour in atherosclerotic peripheral vessels.

Modelling the Impact of Atherosclerosis on Drug Release and Distribution from Coronary Stents

Although drug-eluting stents (DES) are now widely used for the treatment of coronary heart disease, there remains considerable scope for the development of enhanced designs which address some of the limitations of existing devices. The drug release profile is a key element governing the overall performance of DES. The use of in vitro, in vivo, ex vivo, in silico and mathematical models has enhanced understanding of the factors which govern drug uptake and distribution from DES. Such work has identified the physical phenomena determining the transport of drug from the stent and through tissue, and has highlighted the importance of stent coatings and drug physical properties to this process. However, there is limited information regarding the precise role that the atherosclerotic lesion has in determining the uptake and distribution of drug. In this review, we start by discussing the various models that have been used in this research area, highlighting the different types of information they can provide. We then go on to describe more recent methods that incorporate the impact of atherosclerotic lesions.

Local delivery of paclitaxel in the treatment of peripheral arterial disease

BACKGROUND

Despite advancements from balloon angioplasty to drug-eluting stents, primary patency rates after endovascular revascularization of peripheral artery disease have remained inferior compared to surgery. Endovascular revascularization has been limited by restenosis and mechanical stent failure. Thus, there is increased research into other nonstent-based local drug delivery modalities, which can provide an active drug to inhibit restenosis focally and avoid the risk of systemic adverse effects.

METHODS

This review will summarize the unique properties of paclitaxel and studies on paclitaxel local delivery for the treatment of peripheral artery disease. A MEDLINE search for relevant peer-reviewed scientific literature published in English was conducted. Search terms included but were not limited to paclitaxel pharmacodynamics, paclitaxel local drug delivery, and drug eluting balloons, with a focus on the use of paclitaxel in the context of coronary and peripheral vascular disease.

RESULTS

The primary search produced 182 results of which 51 papers were relevant. Of the 51 relevant papers, 27 were original research papers and 24 were either review papers, commentary or opinion papers.

CONCLUSIONS

Paclitaxel has several chemical properties, which make it ideal for local drug delivery including its hydrophobicity, ability to concentrate into the arterial intima layer and prolonged effect on cells even after brief exposure periods. Local delivery of paclitaxel via injection catheters, balloon catheters and coated balloons has shown encouraging results in terms of efficacy and safety in small-scale animal and clinical studies. Additional preclinical and clinical studies are needed to determine the long-term efficacy and safety of these treatments in humans.

Controlled release for local delivery of drugs: barriers and models

Controlled release systems are an effective means for local drug delivery. In local drug delivery, the major goal is to supply therapeutic levels of a drug agent at a physical site in the body for a prolonged period. A second goal is to reduce systemic toxicities, by avoiding the delivery of agents to non-target tissues remote from the site. Understanding the dynamics of drug transport in the vicinity of a local drug delivery device is helpful in achieving both of these goals. Here, we provide an overview of controlled release systems for local delivery and we review mathematical models of drug transport in tissue, which describe the local penetration of drugs into tissue and illustrate the factors - such as diffusion, convection, and elimination - that control drug dispersion and its ultimate fate. This review highlights the important role of controlled release science in development of reliable methods for local delivery, as well as the barriers to accomplishing effective delivery in the brain, blood vessels, mucosal epithelia, and the skin.

Myocardial drug distribution generated from local epicardial application: potential impact of cardiac capillary perfusion in a swine model using epinephrine

Prior studies in small mammals have shown that local epicardial application of inotropic compounds drives myocardial contractility without systemic side effects. Myocardial capillary blood flow, however, may be more significant in larger species than in small animals. We hypothesized that bulk perfusion in capillary beds of the large mammalian heart not only enhances drug distribution after local release, but also clears more drug from the tissue target than in small animals. Epicardial (EC) drug releasing systems were used to apply epinephrine to the anterior surface of the left heart of swine in either point-sourced or distributed configurations. Following local application or intravenous (IV) infusion at the same dose rates, hemodynamic responses, epinephrine levels in the coronary sinus and systemic circulation, and drug deposition across the ventricular wall, around the circumference and down the axis, were measured. EC delivery via point-source release generated transmural epinephrine gradients directly beneath the site of application extending into the middle third of the myocardial thickness. Gradients in drug deposition were also observed down the length of the heart and around the circumference toward the lateral wall, but not the interventricular septum. These gradients extended further than might be predicted from simple diffusion. The circumferential distribution following local epinephrine delivery from a distributed source to the entire anterior wall drove drug toward the inferior wall, further than with point-source release, but again, not to the septum. This augmented drug distribution away from the release source, down the axis of the left ventricle, and selectively toward the left heart follows the direction of capillary perfusion away from the anterior descending and circumflex arteries, suggesting a role for the coronary circulation in determining local drug deposition and clearance. The dominant role of the coronary vasculature is further suggested by the elevated drug levels in the coronary sinus effluent. Indeed, plasma levels, hemodynamic responses, and myocardial deposition remote from the point of release were similar following local EC or IV delivery. Therefore, the coronary vasculature shapes the pharmacokinetics of local myocardial delivery of small catecholamine drugs in large animal models. Optimal design of epicardial drug delivery systems must consider the underlying bulk capillary perfusion currents within the tissue to deliver drug to tissue targets and may favor therapeutic molecules with better potential retention in myocardial tissue.

Analysis of drug distribution from a simulated drug-eluting stent strut using an in vitro framework

The mechanisms of delivery of anti-proliferative drug from a drug-eluting stent are defined by transport forces in the coating, the lumen, and the arterial wall. Dynamic asymmetries in the localized flow about stent struts have previously been shown to contribute to significant heterogeneity in the spatial distribution of drug in in silico three-compartmental models of stent based drug delivery. A novel bench-top experiment has been created to confirm this phenomena. The experiment simulates drug release from a single stent strut, and then allows visualization of drug uptake into both lumen and tissue domains using optical techniques. Results confirm the existence of inhomogeneous and asymmetric arterial drug distributions, with this distribution shown to be sensitive to the flow field surrounding the strut.

Stent elution rate determines drug deposition and receptor-mediated effects

Drug eluting stent designs abound and yet the dependence of efficacy on drug dose and elution duration remains unclear. We examined these issues within a mathematical framework of arterial drug distribution and receptor binding following stent elution. Model predictions that tissue content linearly tracks stent elution rate were validated in porcine coronary artery sirolimus-eluting stents implants. Arterial content varied for stent types, progressively declining from its Day 1 peak and tracking with rate-limiting drug elution--near zero-order release was three-fold more efficient at depositing drug in the stented lesion than near first-order release. In vivo data were consistent with an overabundance of non-specific sirolimus-binding sites relative to the specific receptors and to the delivered dose. The implication is that the persistence time of receptor saturation and effect is more sensitive to duration of elution than to eluted amount. Consequently, the eluted amount should be sufficiently high to saturate receptors at the target lesion, but dose escalation alone is an inefficient strategy for prolonging the duration of sirolimus deposition. Moreover, receptor saturating drug doses are predicted to be most efficacious when eluted from stents in a constant zero order fashion as this maximizes the duration of elution and receptor saturation.

Modelling intravascular delivery from drug-eluting stents with biodurable coating: investigation of anisotropic vascular drug diffusivity and arterial drug distribution

In-stent restenosis occurs in coronary arteries after implantation of drug-eluting stents with non-uniform restenosis thickness distribution in the artery cross section. Knowledge of the spatio-temporal drug uptake in the arterial wall is useful for investigating restenosis growth but may often be very expensive/difficult to acquire experimentally. In this study, local delivery of a hydrophobic drug from a drug-eluting stent implanted in a coronary artery is mathematically modelled to investigate the drug release and spatio-temporal drug distribution in the arterial wall. The model integrates drug diffusion in the coating and drug diffusion with reversible binding in the arterial wall. The model is solved by the finite volume method for both high and low drug loadings relative to its solubility in the stent coating with varied isotropic-anisotropic vascular drug diffusivities. Drug release profiles in the coating are observed to depend not only on the coating drug diffusivity but also on the properties of the surrounding arterial wall. Time dependencies of the spatially averaged free- and bound-drug levels in the arterial wall on the coating and vascular drug diffusivities are discussed. Anisotropic vascular drug diffusivities result in slightly different average drug levels in the arterial wall but with very different spatial distributions. Higher circumferential vascular diffusivity results in more uniform drug loading in the upper layers and is potentially beneficial in reducing in-stent restenosis. An analytical expression is derived which can be used to determine regions in the arterial with higher free-drug concentration than bound-drug concentration.

Arterial pharmacokinetics of red wine polyphenols: implications for novel endovascular therapies targeting restenosis

Drug-eluting stents (DESs) are endovascular devices that provide controlled release of compounds to interfere with restenosis, an adverse outcome of angioplasty characterized by thickening of the arterial wall. Accumulating evidence suggests that arterial pharmacokinetics determine the biological effect and potential toxicity of stent-based therapeutics. The aim of this study was to examine how drug polarity, drug load, and protein binding influence release from a polymer film and distribution within arterial tissue. The transport and safety profile of resveratrol (RESV) and quercetin (QUER), two red wine polyphenols known to interfere with events in the pathogenesis of restenosis, were compared with paclitaxel (Taxol), a lipophilic drug used in DES. In bovine arteries, RESV showed considerable protein binding and arterial kinetics that were found to mimick Taxol. In contrast, the less lipophilic QUER showed limited tissue distribution. Measured diffusivity of RESV and QUER was coupled with a novel computational method for assessment of biphasic drug release kinetics and arterial drug retention profiles. Modeling revealed that drugs associated with high- and low-protein-binding affinity result in markedly distinct arterial drug profiles. These data underscore the importance of arterial partitioning and propagation of drug within arterial tissue in the rational design of DES coatings.

Current status, challenges and future directions of drug-eluting balloons

For the past 30 years, contemporary coronary and endovascular interventions utilized balloon catheters, bare metal- and drug-eluting stents (DES) to recanalize narrowed vessels. Despite this, the quest for outcome optimization is ongoing for specific lesions and patients. Drug-eluting balloons (DEBs) are among the latest technologies proposed to overcome the limitations of DES, such as stent thrombosis and the dependency on long-term dual antiplatelet therapy. In the large part, DEBs were introduced as a substitute for DES in the treatment of in-stent restenosis and perhaps in certain de novo lesion subsets. DEBs have been tested in several clinical scenarios with encouraging preliminary results. This article will discuss the rationale for developing DEBs, basic concepts and available DEB platforms, along with preclinical studies and clinical experience to support the use of this new technology for endovascular interventions.

Development of an in vitro method for modeling drug release and subsequent tissue drug uptake and deposition in a pulsatile flow network

A novel benchtop model of drug elution and arterial drug deposition following stent implantation has been developed. The model contains a single drug loaded strut and a compartment simulating the vessel wall, housed in a flow chamber under a pulsatile flow regime. Each component has programmable transport properties that can be implemented into a computational model of drug elution. An initial experiment determining the effects of luminal flow on drug deposition patterns was performed. The results show that spatial distribution of drug correlates with areas of low and recirculating flow surrounding the strut. This spatial distribution of drug was shown to be dependent on both transient release behavior and the local flow field surrounding the strut. Furthermore, these results showed that the novel method could be used to study the effects of luminal flow in the presence of single or multiple struts. The method could also be used to explore more complex drug release strategies.

Lesion complexity determines arterial drug distribution after local drug delivery

Though stents are deployed in diseased arteries drug distribution has only been quantified in intact, non-diseased vessels. We correlated steady-state arterial drug distribution with tissue ultrastructure and composition in abdominal aortae from atherosclerotic human autopsy specimens and rabbits with lesions induced by dietary manipulation and controlled injury. Paclitaxel, everolimus, and sirolimus depositions in the human aortae were maximal in the media and scaled inversely with lipid content. Net tissue paclitaxel and everolimus levels were indistinguishable in mildly injured rabbit arteries independent of diet. Yet, serial sectioning of cryopreserved arterial segments demonstrated a differential transmural deposition pattern that was amplified with disease and correlated with the expression of their intracellular targets, tubulin and FKBP-12. Tubulin distribution and paclitaxel binding increased with vascular injury and macrophage infiltration, and were reduced with lipid content. Sirolimus analogs and their specific binding target FKBP-12 were less sensitive to alterations of diet in mildly injured arteries, presumably reflecting a faster transient response of FKBP-12 to injury. The data demonstrate that disease-induced changes in the distribution of drug-binding proteins and interstitial lipid alter the distribution of these drugs, forcing one to consider how disease might affect the evaluation and efficacy of the local release of these and like compounds.

Thrombus causes fluctuations in arterial drug delivery from intravascular stents

Arterial drug concentrations determine local toxicity. As such the emergent safety concerns surrounding drug-eluting stents mandate an investigation of the factors contributing to fluctuations in arterial drug uptake. Drug-eluting stents were implanted into porcine coronary arteries, arterial drug uptake was followed and modeled using 2-dimensional computational drug transport. Arterial drug uptake in vivo occurred faster than predicted by free drug diffusion, thus an alternate, mechanism for rapid transport has been proposed involving carrier-mediated transport. Though there was minimal variation in vivo in release kinetics from stent to stent, arterial drug deposition varied by up to 114% two weeks after stent implantation. The extent of adherent mural thrombus also fluctuated by 113% within 3 days after implantation. The computational drug transport model predicted that focal and diffuse thrombi elevate arterial drug deposition in proportion to the thrombus size by reducing drug washout subsequently increasing local drug availability. Fluctuations in arterial drug uptake are commonly reported. We now explain that variable peristrut thrombus can explain such observations even in the face of a narrow range of drug release from the stent. The mural thrombus effects on arterial drug deposition may be circumvented by forcing slow, rate limiting arterial transport that cannot be further hindered by mural thrombus.

Intravascular drug release kinetics dictate arterial drug deposition, retention, and distribution

Millions of patients worldwide have received drug-eluting stents to reduce their risk for in-stent restenosis. The efficacy and toxicity of these local therapeutics depend upon arterial drug deposition, distribution, and retention. To examine how administered dose and drug release kinetics control arterial drug uptake, a model was created using principles of computational fluid dynamics and transient drug diffusion-convection. The modeling predictions for drug elution were validated using empiric data from stented porcine coronary arteries. Inefficient, minimal arterial drug deposition was predicted when a bolus of drug was released and depleted within seconds. Month-long stent-based drug release efficiently delivered nearly continuous drug levels, but the slow rate of drug presentation limited arterial drug uptake. Uptake was only maximized when the rates of drug release and absorption matched, which occurred for hour-long drug release. Of the two possible means for increasing the amount of drug on the stent, modulation of drug concentration potently impacts the magnitude of arterial drug deposition, while changes in coating drug mass affect duration of release. We demonstrate the importance of drug release kinetics and administered drug dose in governing arterial drug uptake and suggest novel drug delivery strategies for controlling spatio-temporal arterial drug distribution.

Coronary stents: a materials perspective

The objective of this review is to describe the suitability of different biomaterials as coronary stents. This review focuses on the following topics: (1) different materials used for stents, (2) surface characteristics that influence stent-biology interactions, (3) the use of polymers in stents, and (4) drug-eluting stents, especially those that are commercially available.

Diffusion of Alexa Fluor 488-conjugated dendrimers in rat aortic tissue

In this study, the distribution of labeled dendrimers in native and aneurysmal rat aortic tissue was examined. Adult male rats underwent infrarenal aorta perfusion with generation 5 (G5) acetylated Alexa Fluor 488-conjugated dendrimers for varying lengths of time. In a second set of experiments, rats underwent aortic elastase perfusion followed by aortic dendrimer perfusion 7 days later. Aortic diameters were measured prior to and postelastase perfusion, and again on the day of harvest. Aortas were harvested 0, 12, or 24 h postperfusion, fixed, and mounted. Native aortas were harvested and viewed as negative controls. Aortic cross-sections were viewed and imaged using confocal microscopy. Dendrimers were quantified (counts/high-powered field). Results were evaluated by repeated measures ANOVA and Student's t-test. We found that in native aortas, dendrimers penetrated the aortic wall in all groups. For all perfusion times, fewer dendrimers were present as time between dendrimer perfusion and aortic harvest increased. Longer perfusion times resulted in increased diffusion of dendrimers throughout the aortic wall. By 24 h, the majority of the dendrimers were through the wall. Dendrimers in aneurysmal aortas, on day 0 postdendrimer perfusion, diffused farther into the aortic wall than controls. In conclusion, this study documents labeled dendrimers delivered intra-arterially to native rat aortas in vivo, and the temporal diffusion of these molecules within the aortic wall. Increasing perfusion time and length of time prior to harvest resulted in continued dendrimer diffusion into the aortic wall. These preliminary data provide a novel mechanism whereby local inhibitory therapy may be delivered locally to aortic tissue.

Diffusion of bioactive molecules through the walls of the medial tissue-engineered hybrid ePTFE grafts for applications in designs of vascular tissue regeneration

Strategies of better vascular tissue engineering may require delivery of soluble bioactive signals in cell culture medium to the cells in tissue-regenerating constructs. We measured the diffusivity and permeability of model tissue-engineering bioactive molecules such as water and heparin through the walls of both a hybrid ePTFE graft and a porcine carotid artery, a model vascular tissue. While diffusivities of H(3)-water and H(3)-heparin were measured as 3.9 x 10(-) (6) and 1.6 x 10(-) (6) cm(2)/s in the artery, respectively, under diffusional circulation of cell culture medium through the lumens of the carotid arteries, their corresponding permeabilities were 4.7 x 10(-) (5) and 2.0 x 10(-) (5) cm/s. On the other hand, diffusivities of H(3)-water and H(3)-heparin were also measured as 5.1 x 10(-) (6) and 4.7 x 10(-) (6) cm(2)/s, respectively, in the tissue-engineered hybrid ePTFE grafts; their corresponding permeabilities were 5.1 x 10(-) (5) and 3.7 x 10(-) (5) cm/s. The hybrid graft tissues were engineered by replacing the biodegradable, porous poly(lactide-co-glycolide) layers coated on the ePTFE surfaces with smooth muscle cell-derived tissues for 6 weeks. We analyzed the morphologies of the artery and the engineered hybrid ePTFE tissues with scanning electron microscopy and H&E stains. While the artery had its typical structure properties with layers of intima, media and adventitia, the tissue-engineered ePTFE hybrid graft had two layers of engineered tissues on the inner and outer surfaces of the ePTFE. There were no significant differences among the luminal tissue morphologies of the test samples from the effects of diffusion flow applications, with minor changes on their luminal surfaces. The results of water and heparin diffusion experiments indicated that these bioactive molecules were well transported from the cell culture medium to the tissue-engineering cells, enough to support tissue regeneration. We hope that these transport results may elucidate the transport behaviors of soluble nutrient molecules and biological signals through the vascular constructs under tissue engineering processes.

Drug-eluting stents: factors governing local pharmacokinetics

Stent-based drug delivery system is a revolutionary approach to mitigate the negative affects of balloon angioplasty, improve immune responsiveness and prevent hyperplastic growth of smooth muscle in the restenotic state. Its success is therefore empirically associated with effective delivery of potent therapeutics to the target site at a therapeutic concentration, for a sufficient time, and in a biologically active form. However, local delivery with drug-eluting stents imparts large dynamic concentration gradients across tissues that can be difficult to identify, characterize and control. This review explores the factors such as physiological transport forces, drug physicochemical properties, local biological tissue properties and stent design that governs the local pharmacokinetics within the arterial wall by drug-eluting stent. Rational design and optimization of drug-eluting stents for local delivery thus requires a careful consideration of all these factors.

Perivascular Tissue Pharmacokinetics of Dipyridamole

PURPOSE

The tissue diffusivity (D(g)) and partitioning (K) for dipyridamole were determined and a model was developed to examine the relationship between perivascular dose and local dipyridamole tissue concentrations.

METHODS

Experiments were performed using an in vitro perfusion apparatus that recirculated buffer through different graft samples or normal porcine femoral arteries and veins. The grafts or blood vessels were immersed in a compartment containing Krebs-Henseleit (KH) buffer and dipyridamole (30 microg/mL). The recirculating buffer was sampled at multiple time points and dipyridamole was assayed. Estimates of the effective diffusivity (D(g)) and partition coefficient (K) of the drug in the vessel wall were determined and used to simulate dipyridamole tissue concentration after perivascular delivery.

RESULTS

Dipyridamole diffusivity within native femoral veins (D(g) = 3.87 +/- 0.93 x 10(-6) cm2/s) was approximately twice that within femoral arteries (D(g) = 2.06 +/- 0.79 x 10(-6) cm2/s, p < 0.01). Explanted grafts showed the lowest diffusivity. Partition coefficients of femoral arteries (K = 4.11 +/- 0.99) were higher than those of femoral veins (K = 2.05 +/- 0.85, p < 0.01) and explanted graft (K = 0.89 +/- 0.56, p < 0.01).

DISCUSSION

The results demonstrate that local drug kinetics vary greatly for different types of blood vessels and grafts. The pharmacokinetic parameters and resulting computational simulations are helpful in exploring perivascular drug delivery strategies.

Local and systemic drug competition in drug-eluting stent tissue deposition properties

The efficacy of drug-eluting stents (DES) requires delivery of potent compounds directly to the underlying arterial tissue. The commercially available DES drugs rapamycin and paclitaxel bind specifically to their respective therapeutic targets, FKBP12 and polymerized microtubules, while also associating in a more general manner with other tissue elements. As it is binding that provides biological effect the question arises as to whether other locally released or systemically circulating drugs can displace DES drugs from their tissue binding domains. Specific and general binding sites for both drugs are distributed across the media and adventitia with higher specific binding associated with the higher specific binding site densities in the media. The ability of rapamycin and paclitaxel to compete for specific protein binding and general tissue deposition was assessed for both compounds simultaneously and in the presence of other commonly administered cardiac drugs. Drugs classically used to treat standard cardiovascular diseases, such as hypertension and hypercoaguability, displace rapamycin and paclitaxel from general binding sites, possibly decreasing tissue reserve capacity for locally delivered drugs. Paclitaxel and rapamycin do not affect the other's binding to their biologically relevant specific protein targets, but can generally displace each other from tissue at three log order molar excess, decreasing arterials loads by greater than 50%. Local competitive binding therefore should not limit the placement of rapamycin and paclitaxel eluting stents in close proximity.

A Thermal Analogy for Modelling Drug Elution from Cardiovascular Stents

Restriction of blood flow by the narrowing or occlusion of arteries is one of the most common presentations of cardiovascular disease. One treatment involves the introduction of a metal scaffold, or stent, designed to prevent recoil and to provide structural stability to the vessel. On the occasions that this treatment is ineffective, failure is usually associated with re-invasion of tissue. This can be prevented by local delivery of drugs which inhibit tissue growth. The drug might be delivered locally in a polymer coating on the stent. This paper develops and explores the use of a thermal analogue of the drug delivery process and the associated three-dimensional convection-diffusion equation to model the spatial and temporal distribution of drug concentration within the vessel wall. This allows the routine use of commercial finite element analysis software to investigate the dynamics of drug distribution, assist in the understanding of the treatment process and develop improved delivery systems. Two applications illustrate how the model might be used to investigate the effects of controllable or measurable parameters on the progression of the process. It is demonstrated that the geometric characteristics of the stent can have significant impact on the homogeneity of the dosing in the vessel wall.

Specific binding to intracellular proteins determines arterial transport properties for rapamycin and paclitaxel

Endovascular drug-eluting stents have changed the practice of medicine, and yet it is unclear how they so dramatically reduce restenosis and how to distinguish between the different formulations available. Biological drug potency is not the sole determinant of biological effect. Physicochemical drug properties also play important roles. Historically, two classes of therapeutic compounds emerged: hydrophobic drugs, which are retained within tissue and have dramatic effects, and hydrophilic drugs, which are rapidly cleared and ineffective. Researchers are now questioning whether individual properties of different drugs beyond lipid avidity can further distinguish arterial transport and distribution. In bovine internal carotid segments, tissue-loading profiles for hydrophobic paclitaxel and rapamycin are indistinguishable, reaching load steady state after 2 days. Hydrophilic dextran reaches equilibrium in several hours at levels no higher than surrounding solution concentrations. Both paclitaxel and rapamycin bind to the artery at 30-40 times bulk concentration. Competitive binding assays confirm binding to specific tissue elements. Most importantly, transmural drug distribution profiles are markedly different for the two compounds, reflecting, perhaps, different modes of binding. Rapamycin, which binds specifically to FKBP12 binding protein, distributes evenly through the artery, whereas paclitaxel, which binds specifically to microtubules, remains primarily in the subintimal space. The data demonstrate that binding of rapamycin and paclitaxel to specific intracellular proteins plays an essential role in determining arterial transport and distribution and in distinguishing one compound from another. These results offer further insight into the mechanism of local drug delivery and the specific use of existing drug-eluting stent formulations.

Dose model for stent-based delivery of a radioactive compound for the treatment of restenosis in coronary arteries

Radiolabeled drug-eluting stents have been proposed recently as a novel method to potentially reduce restenosis in coronary arteries. A P-32 labeled oligonucleotide (ODN) loaded on a polymer coated stent is slowly released in the arterial wall to deliver a therapeutic dose to the target tissue. However, the relatively low proportion of drugs transferred to the arterial wall (<2%-5% typically) raises questions about the degree to which radiolabeled drugs eluted from the stent can contribute to the total radiation dose delivered to tissues. A three-dimensional diffusion-convection transport model is used to model the transport of a hydrophilic drug released from the surface of a stent to the arterial media. Large drug concentration gradients are observed near the stent struts giving rise to a nonuniform radiation activity distribution for the drug in the tissues as a function of time. A voxel-based kernel convolution method is used to calculate the radiation dose rate resulting from this activity build-up in the arterial wall based on the medical internal radiation dose formalism. Measured residence time for the P-32 ODN in the arterial wall and at the stent surface obtained from animal studies are used to normalize the results in terms of absolute dose to tissue. The results indicate that radiation due to drug eluted from the stent contributes only a small fraction of the total radiation delivered to the arterial wall, the main contribution coming from the activity that remains embedded in the stent coating. For hydrophilic compounds with rapid transit times in arterial tissue and minimal binding interactions, the activity build-up in the arterial wall contributes only a small fraction to the total dose delivered by the P-32 ODN stent. For these compounds, it is concluded that radiolabeled drug-eluting stent will not likely improve the performance of radioactive stents for the treatment of restenosis. Also, variability in the delivery efficacy of drug delivery devices makes accurate dosimetry difficult and the drug washout in the systemic circulatory system may yield an unnecessary activity build-up and dose to healthy organs.

New strategies to prevent restenosis

The Holy Grail of cardiovascular pharmacology has been the search for an effective therapy targeting restenosis after angioplasty and/or intra-arterial stenting. The failure of promising therapeutics in clinical trials underscores the complexity and redundancy of the signaling cascades regulating mitogenesis and fibrogenesis. Novel therapeutic modalities have potential to target dysfunctional signaling elements directly in vascular smooth muscle cells. Significant progress in the treatment against restenosis will require the exploitation and cross-fertilization of developments in the fields of pharmacology, bioengineering, genetics, and molecular biology. Collaboration among researchers in these fields will be essential.

Heparin release from slippery-when-wet guide wires for intravascular use

Thin metallic wires with an adherent hydrophilic/ lubricious polymeric coating were manufactured in a new extrusion-like procedure. This procedure is part of a novel and efficient way of assembling lubricious guide wires for intravascular interventions, such as percutaneous transluminal angioplasty. It is reported that heparin can readily be incorporated in the hydrophilic coating. A set of heparin-containing guidewire models was made and studied in detail. This showed that (i). immersion of the guide-wire models in an aqueous environment leads to release of heparin from their surface; (ii). the presence of heparin in the coating does not impede the lubricity of the coils; (iii). addition of stearic acid in the coating, next to heparin, does not influence the lubricity of the guide-wire models. Two different charges of heparin (designated heparin-low and heparin-high) were incorporated in the coating. It is discussed that release of heparin from the surface of medical devices (e.g. guide wires and catheters) is much more effective than systemic heparinization, basically because dissolved heparin molecules have a much larger probability of simply passing a medical device's surface (axial convection) rather than contacting it (radial diffusion).

Impact of transport and drug properties on the local pharmacology of drug-eluting stents

Drugs released from stents are driven by physiological transport forces, principally solvent-driven flow (convection) and random molecular agitation (diffusion). The relative strength of these two forces determines drug penetration and distribution in the arterial wall. Drug physicochemical factors can induce critical modulations to the primary distribution, both transiently and at steady state. Hydrophobic interactions and nonspecific binding, for example, can both result in tissue drug concentrations severalfold above administered concentration. Drug interaction with native proteins may also interfere with drug transfer at the stent-artery interface. These transport forces and tissue interactions can induce local drug concentrations even at steady state to vary by one or more orders of magnitude over the span of a few cells. To account for significant local variations in drug concentrations following stent-based delivery, rational design of vascular delivery systems requires consideration of drug distribution and tissue interactions on a local, continuum basis. Continuum analysis adapts traditional pharmacokinetics to the local environment by supplementing discrete global parameters of drug content with continuous local values of concentration, transport and binding. The interplay of these parameters with local flux conditions and drug and tissue properties defines the local drug distribution in space and over time. This type of analysis may well become increasingly relevant given the trend toward stent-based drug therapy in cardiovascular care.

Local gene delivery to the vessel wall

This review will provide an overview of delivery strategies that are being evaluated for vascular gene therapy. We will limit our discussion to those studies that have been demonstrated, utilizing in vivo model systems, to limit post-interventional restenosis. We also discuss the efficacy of the vectors and methods currently being used to transfer genetic material to the vessel wall. The efficiency of these techniques is a critical issue for the successful application of gene therapy.

Carrier proteins determine local pharmacokinetics and arterial distribution of paclitaxel

The growing use of local drug delivery to vascular tissues has increased interest in hydrophobic compounds. The binding of these drugs to serum proteins raises their levels in solution, but hinders their distribution through tissues. Inside the arterial interstitium, viscous and steric forces and binding interactions impede drug motion. As such, this might be the ideal scenario for increasing the amount of drug delivered to, and residence time within, arterial tissues. We quantified carrier-mediated transport for paclitaxel, a model hydrophobic agent with potential use in proliferative vascular diseases, by determining, in the presence or absence of carrier proteins, the maximum concentration of drug in aqueous solution, the diffusivity in free solution, and the diffusivity in arterial tissues. Whereas solubility of paclitaxel was raised 8.1-, 21-, and 57-fold by physiologic levels of alpha(1)-acid glycoproteins, bovine serum albumin, and calf serum over that in protein-free solution, diffusivity of paclitaxel in free solution was reduced by 41, 49, and 74%, respectively. When paclitaxel mixed in these solutions was applied to arteries both in vitro and in vivo, drug was more abundant at the tissue interface, but protein carriers tended to retain drug in the lumen. Once within the tissue, these proteins did not affect the rate at which drug traverses the tissue because this hydrophobic drug interacted with the abundant fixed proteins and binding sites. The protein binding properties of hydrophobic compounds allow for beneficial effects on transvascular transport, deposition, and distribution, and may enable prolonged effect and rationally guide local and systemic strategies for their administration.

Endothelial implants provide long-term control of vascular repair in a porcine model of arterial injury

Cell culture and animal data support the role of endothelial cells and endothelial-based compounds in regulating vascular repair after injury. We describe a long-term study in pigs in which the biological and immunological responses to endothelial cell implants were investigated 3 months after angioplasty, approximately 2 months after the implants have degraded. Confluent porcine or bovine endothelial cells grown in polymer matrices were implanted adjacent to 28 injured porcine carotid arteries. Porcine and bovine endothelial cell implants significantly reduced experimental restenosis compared to control by 56 and 31%, respectively. Host humoral responses were investigated by detection of an increase in serum antibodies that bind to the bovine or porcine cell strains used for implantation. A significant increase in titer of circulating antibodies to the bovine cells was observed after 4 days in all animals implanted with xenogeneic cells. Detected antibodies returned to presurgery levels after Day 40. No significant increase in titer of antibodies to the porcine cells was observed during the time course of the experiment in animals implanted with porcine endothelial cells. No implanted cells, Gelfoam, or focal inflammatory reaction could be detected histologically at any of the implant sites at 90 days. These data suggest that tissue-engineered endothelial cell implants may provide long-term control of vascular repair after injury, rather than simply delaying lesion formation and that allogeneic implants are able to provide a greater benefit than xenogeneic implants.

Complete inhibition of intimal hyperplasia by perivascular delivery of paclitaxel in balloon-injured rat carotid arteries

PURPOSE

To determine whether perivascular delivery of paclitaxel prevents luminal narrowing after balloon injury by inhibiting intimal hyperplasia.

MATERIALS AND METHODS

Immediately after balloon injury of the entire left common carotid artery, three slow-release formulations of paclitaxel or control formulations without drug were applied around a distal segment of the artery. The noninjured right carotid arteries were evaluated as a control. The animals were maintained for 14 and 28 days (n = 5 in each group at each time interval). Histology, immunohistochemistry, and morphometric analysis were performed.

RESULTS

Injured nontreated arteries exhibited a pronounced intimal hyperplasia (0.185 +/- 0.01 mm2 at 14 days and 0.189 +/- 0.01 mm2 at 28 days) and a marked reduction in luminal area (44% at 14 days and 43% at 28 days). Medial area and the number of medial cells increased by 44% and 45%, respectively, at 14 days, and by 22% and 37%, respectively, at 28 days. Injured arteries treated with perivascular paclitaxel did not show any intimal hyperplasia, and luminal area was increased in five of six groups and was unchanged in one group. These arteries had an increased medial area but they had fewer medial cells than noninjured arteries. Injured arteries treated with control implants without paclitaxel exhibited intimal hyperplasia and luminal narrowing.

CONCLUSION

Perivascular slow release of paclitaxel totally inhibits intimal hyperplasia and prevents luminal narrowing after balloon injury. Because of its efficacy, perivascular paclitaxel represents a possible approach for prevention of restenosis in humans.

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.

Kinetics of release of heparin from alginate hydrogel

PURPOSE

Injected sodium alginate may be a useful perivascular drug delivery vehicle. This study was performed to determine the release rates of heparin from sodium alginate hydrogels cross-linked with varying amounts of calcium gluconate.

MATERIALS AND METHODS

Six hydrogels, composed of 0.16 mEq sodium alginate and 4,000 units unfractionated heparin, were cross-linked with calcium gluconate to yield ion equivalence (IE) ratios (calcium:alginate) of 0.2, 0.4, 0.58, 0.8, 1.0, or 1.2. Two milliliters of normal saline was placed on top of each gel and allowed to remain in contact for up to 10 days. At set time intervals, the amount of heparin in the eluent was determined with use of high-performance liquid chromatography.

RESULTS

Gels with 0.2 and 0.4 IE were partially liquid at 24 hours; the other gels solidified within 10 minutes. The 0.58 IE gel was slowest to solidify but immobilized the most heparin and released heparin slowest over 10 days. At 10 days, between 5.5% and 9.8% of the heparin immobilized was retained in the gel.

CONCLUSION

This hydrogel shows promise as a vehicle for in vivo perivascular heparin delivery. The 0.58:1 IE ratio hydrogel has slowest release rate and the greatest immobilization despite its longer cross-linking time.

Metabolism of VLDL is increased in streptozotocin-induced diabetic rat hearts

In streptozotocin (STZ)-induced diabetic rats, we previously showed an increased heparin-releasable (luminal) lipoprotein lipase (LPL) activity from perfused hearts. To study the effect of this enlarged LPL pool on triglyceride (TG)-rich lipoproteins, we examined the metabolism of very-low-density lipoprotein (VLDL) perfused through control and diabetic hearts. Diabetic rats had elevated TG levels compared with control. However, fasting for 16 h abolished this difference. When the plasma lipoprotein fraction of density <1.006 g/ml from fasted control and diabetic rats was incubated in vitro with purified bovine or rat LPL, VLDL from diabetic animals was hydrolyzed as proficiently as VLDL from control animals. Post-heparin plasma lipolytic activity was comparable in control and diabetic animals. However, perfusion of control and diabetic rats with heparinase indicated that diabetic hearts had larger amounts of LPL bound to heparan sulfate proteoglycan-binding sites. [(3)H]VLDL obtained from control rats, when recirculated through the isolated heart, disappeared at a significantly faster rate from diabetic than from control rat hearts. This increased VLDL-TG hydrolysis was essentially abolished by prior perfusion of the diabetic heart with heparin, implicating LPL in this process. These findings suggest that the enlarged LPL pool in the diabetic heart is present at a functionally relevant location (at the capillary lumen) and is capable of hydrolyzing VLDL. This could increase the delivery of free fatty acid to the heart, and the resultant metabolic changes could induce the subsequent cardiomyopathy that is observed in the chronic diabetic rat.

Insights into the molecular pathogenesis of atherosclerosis and therapeutic strategies using gene transfer

Gene therapy for the treatment of atherosclerosis and related diseases has shown its potential in animal models and in the first human trials. Gene transfer to the vascular system can be performed both via intravascular and extravascular periadventitial routes. Intravascular gene transfer can be done with several types of catheters under fluoroscopic control. Extravascular gene transfer, on the other hand, provides a well-targeted gene delivery route available during vascular surgery. It can be done with direct injection or by using perivascular cuffs or surgical collagen sheets. Ex vivo gene delivery via transfected smooth muscle cells or endothelial cells might be useful for the production of secreted therapeutic compounds. Gene transfer to the liver has been used for the treatment of hyperlipidemia. The first clinical trials for the induction of therapeutic angiogenesis in ischemic myocardium or peripheral muscles with VEGF or FGF gene transfer are under way and preliminary results are promising. VEGF has also been used for the prevention of postangioplasty restenosis because of its capability to induce endothelial repair and production of NO and prostacyclin. However, further basic research is needed to fully understand the pathophysiological mechanisms involved in conditions related to atherosclerosis. Also, further development of gene transfer vectors and gene delivery techniques will improve the efficacy and safety of human gene therapy.

Analysis of adenoviral transport mechanisms in the vessel wall and optimization of gene transfer using local delivery catheters

Local delivery devices have been used for adenovirus-mediated gene transfer to the arterial wall for the potential treatment of vascular proliferative diseases. However, low levels of adenoviral gene expression in vascular smooth muscle cells may pose a serious limitation to the success of these procedures in the clinic. In this study, we examined the mechanisms controlling adenoviral transport to the vessel wall, using both hydrogel-coated and infusion-based local delivery catheters, with the goal of enhancing in vivo gene transfer under clinically relevant delivery conditions. The following delivery parameters were tested in vivo: applied transmural pressure, viral solution volume and concentration, and delivery time. We found that viral particles are transported into the vessel wall in a manner consistent with diffusion rather than pressure-driven convection. Consistent with diffusion, viral concentration was shown to be the key variable for viral transport in the vessel wall and thus gene expression in vascular smooth muscle cells. A transduction level of 17.8+/-3.2% was achieved by delivering a low volume of concentrated adenoviral beta-galactosidase solution through an infusion balloon catheter at low pressure without an adverse effect on medial cellularity. Under these conditions, effective gene transfer was accomplished within a clinically relevant time frame of 2 min, indicating that longer delivery times may not be necessary to achieve efficient gene transfer.

Periadventitial lacZ gene transfer to pig carotid arteries using a biodegradable collagen collar or a wrap of collagen sheet with adenoviruses and plasmid-liposome complexes

BACKGROUND

Periadventitial gene therapy is a promising alternative for the treatment of stenosis, vessel wall thickening and other complications in vascular surgery.

METHODS

We compared lacZ gene transfer efficiency of DOTMA: DOPE (1:1 w/w) plasmid/liposome complexes and adenoviruses in pig carotid arteries using perivascular delivery with either a collagen collar or a wrap of collagen sheet. Safety of the gene transfer was studied by clinical chemistry, tissue pathology and PCR analysis of lung, liver, kidney, spleen, skeletal muscle and gonads.

RESULTS

Gene transfer efficiency using the periadventitial collar was fourfold higher than using the collagen wrap with adenovirus at 7 days (10.22 +/- 2.96 vs 2.78 +/- 1.28 positive cells/mm2; p = 0.18) and 4.3-fold at 14 days (13.46 +/- 3.49 vs 3.11 +/- 0.88 positive cells/mm2; p = 0.03). Gene transfer efficiency at 7 days with adenovirus was fivefold higher than with the plasmid/liposome complexes both using the collar (10.22 +/- 2.96 vs 2.07 +/- 0.95 positive cells/mm2; p = 0.01) and the collagen wrap (2.78 +/- 1.28 vs 0.45 +/- 0.35 positive cells/mm2; p = 0.03). No lacZ activity was detected in plasmid/liposome transfected arteries at 14 days. In spite of the local gene delivery methods a moderate systemic distribution of the transgene was detected in the major organs by PCR analysis.

CONCLUSIONS

This study shows that: (i) adenovirus delivered with the periadventitial collar or the collagen wrap is well tolerated and may become an efficient new tool in vascular gene therapy, and (ii) gene transfer vector delivered in the periadventitial collar reaches the target tissue more efficiently than the vector in the collagen wrap.

Mechanisms of heparin transport through expanded poly(tetrafluoroethylene) vascular grafts

Thrombosis and neointimal hyperplasia limit the utility of small-caliber artificial vascular grafts. Surface modifications and adjunctive pharmacological therapy might mediate these complications. We examined the mechanisms by which a model vasoactive compound, heparin, transverses porous graft materials and how material modifications alters this drug's transport. The effective permeance of [(3)H]heparin was measured after application of a uniform concentration of drug to either the internal or external surface of the graft and in the presence or absence of pressure-driven physiologic hydraulic flows. Transgraft permeance was equivalent to those observed in normal arteries and, while enhanced by convection, was mediated in major part by diffusion. Peclet numbers under the various conditions examined ranged from 0.05 to 1.2, indicating that diffusive forces were equal to or exceeded convective forces in governing transmural heparin motion. Heparin traversed the graft even when applied from the outer perivascular surface, against adverse hydraulic flows. Modifications of the grafts that included a yarn barrier of spun poly(tetrafluoroethylene) or chemical modification of surface tension energy altered permeances as well. A unifying model for interpretation of these data incorporates the concept of entrapped air and surface tension energy in the graft. These characterizations allow for the design of vascular grafts that are optimized for pharmacotherapy to help prolong graft patency, especially in small-caliber vascular beds.

Tissue concentration of heparin, not administered dose, correlates with the biological response of injured arteries in vivo

Drug activity is often studied in well controlled and characterized cellular environments in vitro. However, the biology of cells in culture is only a part of the tissue behavior in vivo. Quantitative studies of the dose response to drugs in vivo have been limited by the inability to reliably determine or predict the concentrations achieved in tissues. We developed a method to study the dose response of injured arteries to exogenous heparin in vivo by providing steady and predictable arterial levels of drug. Controlled-release devices were fabricated to direct heparin uniformly and at a steady rate to the adventitial surface of balloon-injured rat carotid arteries. We predicted the distribution of heparin throughout the arterial wall by using computational simulations of intravascular drug binding and transport, and we correlated these concentrations with the biologic response of the tissues. This allowed the estimation of the arterial concentration of heparin required to maximally inhibit intimal hyperplasia after injury in vivo, 0.3 mg/ml. This estimation of the required concentration of drug seen by a specific tissue is independent of the route of administration and holds for all forms of drug release. In this way we may now be able to evaluate the potential of widely disparate forms of drug release and to finally create some rigorous criteria by which to guide the development of particular delivery strategies for local diseases.

A new experimental approach in endothelium-dependent pharmacological investigations on isolated porcine coronary arteries mounted for impedance planimetry

1. The aim of this study was to investigate whether the balloon-based impedance planimetry technique could be a useful tool in endothelium-dependent investigations. 2. Porcine large coronary arteries contracted with prostaglandin F2alpha (PGF2alpha, 10 microM) did not relax to bradykinin (0.1 nM - 0.1 microM), but did relax to sodium nitroprusside (SNP, 10 microM). However, after eversion of the segments, bradykinin induced relaxations with pD2 values and maximal responses of 8.78+/-0.09 and 75+/-2% (n=6), respectively. 3. Incubation with captopril (1 microM) did not reveal a relaxation to bradykinin in the normal vessel configuration and had no influence on the concentration-relaxation relationship in everted segments. 4. Lowering the luminal pressure in contracted segments from 131+/-5 mmHg (isometric, n=5) to 60 mmHg (isobaric, n=5) did not facilitate the action of bradykinin. 5. Eversion of segments did not influence the concentration-response relationship for K+ (4.7 - 125 mM), PGF2alpha (0.3 - 30 microM), and SNP (30 nM - 30 microM), although the time-courses of responses were faster when the agents were added from the intimal compared to the adventitial side of the preparation. 6. In the same everted segment contracted with PGF2alpha, the concentration-response relationship for bradykinin was not different under isometric and isobaric conditions. 7. These results indicate that, (1) reduced endothelium-dependent relaxations to adventitially administered substances can be ascribed to a diffusion barrier in the vessel wall, while enzymatic degradation, luminal pressure and precontractile responses seem not to play a role, (2) impedance planimetry applied to everted cylindrical segments could be a useful experimental approach in pharmacological studies of endothelium-dependent responses under isobaric and isometric conditions.

Measurement of drug distribution in vascular tissue using quantitative fluorescence microscopy

Quantitative tools to assess vascular macromolecular distributions have been limited by low signal-to-noise ratios, reduced spatial resolution, postexperimental motion artifact, and the inability to provide multidimensional drug distribution profiles. Fluorescence microscopy offers the potential of identifying exogenous compounds within intact tissue by reducing autofluorescence, the process by which endogenous compounds emit energy at the same wavelength as fluorescent labels. A new technique combining fluorescence microscopy with digital postprocessing has been developed to address these limitations and is now described in detail. As a demonstration, histologic cross-sections of calf carotid arteries that had been loaded endovascularly with FITC-Dextran (20 kD) ex vivo were imaged at two different locations of the electromagnetic spectrum, one exciting only autofluorescent structures and the other exciting both autofluorescent elements and exogenous fluorescent labels. The former image was used to estimate the autofluorescence in the latter. Subtraction of the estimated autofluorescence resulted in an autofluorescence-corrected image. A standard curve, constructed from arteries that were incubated until equilibrium in different bulk phase concentrations of FITC-Dextran, was used to convert fluorescent intensities to tissue concentrations. This resulted in a concentration map with spatial resolution superior to many of the previous methods used to quantify macromolecular distributions. The transvascular concentration profiles measured by quantitative fluorescence microscopy compared favorably with those generated from the proven en face serial sectioning technique, validating the former. In addition, the fluorescence method demonstrated markedly increased spatial resolution. This new technique may well prove to be a valuable tool for elucidating the mechanisms of macromolecular transport, and for the rational design of drug delivery systems.