Arterial paclitaxel distribution and deposition

Drug-eluting stents studies in mice: Do we need atherosclerosis to study restenosis?

In 2001, the first human study with drug-eluting stents (DES) was published showing a nearly complete abolition of restenosis by using a sirolimus-eluting stent. This success was very encouraging to test new compounds in combination with the DES platform. Nevertheless, several other anti-restenotic compounds have been used in human clinical trials with disappointing outcomes. Little is known concerning potential adverse effects on vessel wall integrity and (re)healing, atherosclerotic lesion formation, progression, and plaque stability of these DES. Although efficacy and safety need to be determined clinically, preclinical testing of candidate drugs in well-defined animal models is extremely helpful to gain insight into the basic biological responses to candidate compounds. Here, we discuss and report an animal model which enables rapid screening of candidate drugs for DES on an atherosclerotic background. The results from drug testing using this novel model could help to quickly and cost-effectively establish the dose range of candidate drugs with reasonable potential for DES.

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.

Paclitaxel-eluting stents in coronary artery disease

PURPOSE

Clinical information regarding paclitaxel-eluting coronary artery stents is reviewed.

SUMMARY

Restenosis is a significant complication of percutaneous coronary intervention. Coronary artery stenting has reduced restenosis compared with traditional balloon angioplasty, although restenosis still occurs with bare-metal coronary artery stents. The pathogenesis of in-stent restenosis is believed to involve smooth-muscle-cell proliferation and migration in response to vessel injury. A neointimal layer of extracellular matrix and collagen forms, which may impinge on the vessel lumen. Paclitaxel inhibits vascular smooth-muscle-cell proliferation and reduces neointimal mass. Local delivery of paclitaxel through a coronary stent has been shown to reduce restenosis rates and percent diameter stenosis and to produce other angiographic benefits compared with bare-metal stents. Fewer major adverse coronary events are seen with paclitaxel-eluting stents, predominantly because of a reduction in the need for target-vessel revascularization with minimal impact on rates of mortality and myocardial infarction (MI). The Taxus Express(2) stent, the only approved paclitaxel-eluting stent in the United States, costs about three times as much as a bare-metal stent. Cost-effectiveness analyses are needed to determine if the Taxus stent is cost-effective in clinical practice.

CONCLUSION

Paclitaxel-eluting stents reduce the rates of restenosis and target-vessel revascularization compared with bare-metal stents and have comparable effects on mortality and MI rates.

Development of a local perivascular paclitaxel delivery system for hemodialysis vascular access dysfunction: polymer preparation and in vitro activity

Hemodialysis vascular access dysfunction (HVAD) is currently a huge clinical problem. The major cause of HVAD is venous stenosis (as a result of venous neointimal hyperplasia) which leads to thrombosis in polytetrafluoroethylene dialysis access grafts and fistulae. Despite the magnitude of the clinical problem there are currently no effective therapeutic interventions for this condition. In an attempt to reduce the morbidity associated with HVAD, we have developed and validated a local perivascular paclitaxel release system for use in a pig model of arteriovenous graft stenosis. Ethylene vinyl acetate polymers with 5% paclitaxel were formulated. The release profile of paclitaxel was then manipulated to maximize its biological impact in the in vivo situation. In vitro experiments were performed to confirm that the paclitaxel released from the polymer was biologically active against cell types that were similar to those present in the in vivo lesion of neointimal hyperplasia. Our results demonstrate that the paclitaxel polymer wraps which we have developed are mechanically stable with a burst release phase followed by a slower continuous release phase. The paclitaxel released from these polymeric wraps retains its physicochemical and biological properties and is able to inhibit the proliferation of smooth muscle cells, endothelial cells and fibroblasts in vitro. We believe that these paclitaxel-loaded polymeric wraps could be ideally suited for perivascular drug delivery in the context of dialysis access grafts and fistulae.

Effect of Paclitaxel and Mesenchymal Stem Cells Seeding on Ex Vivo Vascular Endothelial Repair and Smooth Muscle Cells Growth

Late thrombosis and neointima proliferation after paclitaxel-eluting stents implanting may be related to delayed endothelial cells (ECs) regeneration. This study was to investigate whether mesenchymal stem cells (MSCs) seeding can accelerate endothelial repair and attenuate late smooth muscle cells (SMCs) proliferation after paclitaxel intervention. An ex vivo model of endothelium repair was developed in which rabbit smooth muscle cells were inoculated in the upper chamber and rabbit endothelial cells/human mesenchymal stem cells in the lower chamber of a co-culture system. Paclitaxel (10 nmol/L, 20 min) inhibited smooth muscle cell growth of the confluent endothelial cell group during the observed period. However, increased smooth muscle cells growth was observed in the proliferative endothelial cells group 10 days after paclitaxel intervention. Mesenchymal stem cell seeding inhibited late smooth muscle cell growth incompatible with the effect of proliferative endothelial cells. However, no inhibition on smooth muscle cell growth was observed with mesenchymal stem cell seeding in comparison to the effect of confluent endothelial cells. No vWF but Flk-1 protein was observed in the 25.71% of mesenchymal stem cells after having been co-cultured with rabbit endothelial cells for 5 days. These results indicate that late smooth muscle cell proliferation is closely related to the delayed endothelial cells regeneration after paclitaxel application. Mesenchymal stem cell seeding partly attenuates the late smooth muscle cell proliferation. Mesenchymal stem cells co-cultured with mature endothelial cells have the ability to differentiate toward endothelial cells.

Triple-coated stents (Hirudin/Iloprost/Paclitaxel): an in vitro approach for characterizing the antiproliferative potential of each individual compound

BACKGROUND

Hirudin (H)/iloprost (I)/paclitaxel (P)-coated stents represent a multifactorial approach to reducing the proliferative response caused by ballooning and stenting. The study presented compares the net effect of each individual compound of HIP-coated stents with the summed effect of the compounds in the stent coating.

METHODS AND RESULTS

For proliferation prescreening studies, human coronary smooth muscle cells were incubated with H (0.005-500 microg/ml), I (0.00001-1 microg/ml), and P (0.0001-10 microg/ml). After 5 days, cell number was studied in a cell analyzer system. Secondly, 8-mm stents were coated with (1) HI, (2) HIP-10 microg/20 microg/40 microg (HIP5%/10%/20%), (3) P-40 microg (P), (4) IP-40 microg (IP), and (5) HP-40 microg (HP). After 5 days, the effect on cell proliferation and cytoskeletal structures was studied. No antiproliferative effect was found after incubation with H; significant inhibition was seen after incubation with I (p<0.05) or lipophilically dissolved P (p<0.001). After 5 days incubation with HIP5%-, HIP10%-, HIP20%-, P20%-, IP20%-, and HP20%-coated stents, cell proliferation was inhibited by 55.5% (p<0.05), 61% (p<0.05), 57.9% (p<0.05), 59.5% (p<0.001), 59.8% (p<0.001), and 63.3% (p<0.001), respectively. HI- and HIP-coated stents caused a severe destruction of the cytoskeletal structures smooth muscle alpha-actin and alpha-tubulin; despite the destruction, vital cells could be identified with positive FDA staining.

CONCLUSIONS

Although both lipophilically dissolved P and hydrophilically dissolved I contributed to the antiproliferative effect, no additive effect of the two compounds was detected. In vivo P can be released more easily from the coating material due to the permanent lipophilic contact of the stent struts with the vessel wall. The current study is the first report on a clear and uncomplicated technique to obtain information on the antiproliferative potential of coated stents before large experimental studies are initiated.

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.

Molecular responses of vascular smooth muscle cells to paclitaxel-eluting bioresorbable stent materials

We studied the influence of paclitaxel, eluted from poly(L-lactic acid) (PLLA), on cultured vascular smooth muscle cell (VSMC) proliferation as a model of bioresorbable stent-induced restenosis. We blended paclitaxel in cast PLLA films (P-PLLA), demonstrating controlled release of the drug, then studied VSMC adhesion, proliferation, and gene expression profiles. No difference in cell adhesion was found between P-PLLA and PLLA controls (105 +/- 12% of PLLA controls). However, P-PLLA significantly reduced VSMC proliferation (40 +/- 15% of PLLA controls, p < 0.05). Using cDNA microarray technology, we identified major effects of P-PLLA, including: upregulation of genes related to apoptosis, anti-proliferation and antioxidation; and suppression of cell cycle regulators and cell survival markers. The expression patterns indicate that P-PLLA regulates gene expression and cell functions via new pathways, including receptor tyrosine kinase (RTKs), mitogen-activated protein kinase (MAPKs), and protein kinase (PKs, e.g., PKA) pathways, in addition to the stabilization of polymerized-microtubules.

Paclitaxel-Eluting Stents: Are They All Equal? An Analysis of Six Randomized Controlled Trials in De Novo Lesions of 3,319 Patients

In Germany, four different drug eluting stents (DES) systems are currently commercially available. Whereas sirolimus has been clinically tested in only a single type of stent with a single type of coating in only a single dose, paclitaxel has been tested on various stent designs, in various dose densities, and in various release formulations with or without a polymer carrier. Therefore, the question arises: are all paclitaxel stents equally safe and effective? Six clinical randomized trials investigated the safety and efficacy of paclitaxel-eluting stents in patients with de-novo lesions: TAXUS-I (61 pats), TAXUS-II (536 pats), ASPECT (177 pats), ELUTES (190 pats), DELIVER-I (1041 pats) and TAXUS-IV (1314 pats). In the TAXUS-series, paclitaxel released from the stent was controlled by the Translute polymer. In the other studies, however, no polymer carrier was used. In TAXUS-I, II & IV, the dose density of 1 microg/mm2 significantly reduced angiographic parameters of restenosis and improved clinical outcomes. In ASPECT and ELUTES there was a dose-dependent effect on angiographic parameters of restenosis with the best results for a paclitaxel dose density of approximately 3.0 microg/mm2. Clinical outcomes at 6 and 12 months, however, were not improved in these studies without coating. The studies unanimously show that the paclitaxel-eluting stents are safe, if clopidogrel is added to ASA for 3 to 6 months. The safety of paclitaxel-eluting stents is independent of the stent design, the dose density and the presence or absence of a polymer carrier system. For paclitaxel-eluting stents using a polymer carrier, the dose density of 1 microg/mm2 is highly effective, whereas for paclitaxel-eluting stents without a polymer carrier, the minimal effective dose density is much higher (3 microg/mm2). Despite their improvement of angiographic parameters, paclitaxel-eluting stents without a polymer carrier did not demonstrate a positive effect on clinical outcome. In contrast, polymer-based paclitaxel elution produced significant clinical benefit.

Addition of paclitaxel to contrast media prevents restenosis after coronary stent implantation

OBJECTIVES

The present study was designed to test the efficacy of paclitaxel added to the contrast agent iopromide in the prevention of restenosis.

BACKGROUND

Contrast media adhere to the coronary vessel wall for some seconds after injection. Such a layer of contrast agent could serve as a matrix for antiproliferative drugs.

METHODS

Thirty-four stents were implanted into the left anterior descending and circumflex coronary arteries of 17 pigs, using a 1.2:1.0 overstretch ratio. The unsupplemented contrast agent iopromide-370 was used as a control; the treatment groups were treated with 80 ml intracoronary iopromide plus either 100 or 200 mumol/l paclitaxel, or 80 ml intravenous iopromide plus 200 mumol/l paclitaxel. Quantitative angiography and histomorphometry were used to assess comparable baseline parameters between the treatment groups.

RESULTS

A short time incubation (3 min) almost completely inhibited vascular smooth muscle cell proliferation, sustained for up to 12 days. Whereas intravenous paclitaxel had no effect, intracoronary application of paclitaxel reduced the diameter stenosis from 55 +/- 13% to 29 +/- 18% and 13 +/- 12%. Late lumen loss dropped from 1.94 +/- 0.35 mm under the control condition to 1.19 +/- 0.55 mm with 100 mumol/l paclitaxel and to 0.82 +/- 0.54 mm with 200 mumol/l paclitaxel. Histomorphometry revealed a corresponding dose-dependent reduction of the neointimal area and restenosis by intracoronary iopromide paclitaxel. Assessment of left ventricular function and myocardial histology revealed no adverse effects of intracoronary paclitaxel application.

CONCLUSIONS

This study provides evidence that intracoronary application of a taxane dissolved in a contrast medium profoundly inhibits in-stent restenosis. This novel, widely feasible approach may be suited for the prevention of restenosis in a broad spectrum of interventional treatment regimens.

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.

Binding and retention of polycationic peptides and dendrimers in the vascular wall

Extracellular matrix (ECM) of tissues, vascular tissue in particular, contains a high concentration of negatively charged glycosaminoglycans (GAGs), which are involved in the regulation of cell motility, cell proliferation and the regulation of enzyme activities. Previously, we have shown that the vascular ECM is capable of binding an extremely high concentration of positively charged molecules, such as polylysine. Vascular ECM can be used therefore as a substrate for binding and retention of drugs delivered intravascularly, if these drugs are endowed with an ability to bind to the vascular ECM. In this study, we evaluated a number of positively charged molecules as potential affinity vehicles for delivery of drugs to the vascular ECM. We labelled the molecules of interest with fluorescence and compared them ex vivo in terms of binding and retention in the de-endothelialised rat carotid artery after intravascular delivery under pressure. High molecular weight polylysine (84 kDa) and polyamidoamine (PAMAM) dendrimers accumulated in the wall of the artery up to a concentration of 10 mg/ml and were not washed away significantly after 4 h of perfusion of the artery. A 24-mer peptide containing a consensus sequence for binding to GAGs (ARRRAARA)(3), 2.7 kDa, was comparable to high molecular weight polylysine and dendrimers in terms of binding and retention. A 14-mer GAG-binding peptide from vitronectin and low molecular weight polylysine, 3 kDa, accumulated in the vascular wall up to about 3 mg/ml and was washed away after 30 min of perfusion. A 10-mer consensus GAG-binding peptide did not bind significantly to the vascular tissue. We conclude that the consensus 24-mer GAG-binding peptide is by far superior to polylysine of a similar molecular weight in terms of binding to vascular tissue, and can provide high accumulation and long-term retention of a low molecular weight compound (fluorescein, as a model molecule) in the vascular wall. Rationally designed GAG-binding peptides can be useful as affinity vehicles for targeting drugs to the vascular ECM.

Biodegradable nanoparticles for drug and gene delivery to cells and tissue

Biodegradable nanoparticles formulated from poly (D,L-lactide-co-glycolide) (PLGA) have been extensively investigated for sustained and targeted/localized delivery of different agents including plasmid DNA, proteins and peptides and low molecular weight compounds. Research about the mechanism of intracellular uptake of nanoparticles, their trafficking and sorting into different intracellular compartments, and the mechanism of enhanced therapeutic efficacy of nanoparticle-encapsulated agent at cellular level is more recent and is the primary focus of the review. Recent studies in our laboratory demonstrated rapid escape of PLGA nanoparticles from the endo-lysosomal compartment into cytosol following their uptake. Based on the above mechanism, various potential applications of nanoparticles for delivery of therapeutic agents to the cells and tissue are discussed.

Pharmacokinetics of paclitaxel administered by hyperthermic retrograde isolated lung perfusion techniques

OBJECTIVE

Although paclitaxel is widely used as a systemic agent for the treatment of solid tumors, limited information is available concerning administration of this taxane by regional techniques. The present study was undertaken to evaluate the pharmacokinetics and acute toxicity of paclitaxel administered by hyperthermic retrograde isolated lung perfusion techniques to ascertain its potential for the regional therapy of unresectable pulmonary neoplasms.

METHODS

Adult sheep underwent 90 minutes of retrograde isolated lung perfusion with escalating doses of paclitaxel and moderate hyperthermia using a protein-free, oxygenated extracorporeal circuit and a steady perfusion pressure of 14 to 16 mm Hg. An additional animal received paclitaxel by means of 1-hour central venous infusion. Paclitaxel concentrations in lung tissues, perfusates, and systemic circulation were determined by high-performance liquid chromotography techniques. Cytotoxicity of paclitaxel in cancer cells and in normal human bronchial epithelial cells was evaluated in vitro using 4, 5-dimethylthiazo-2-yl-25-dipagnyl tetrazolium bromide assays. Lung tissues were examined by hematoxylin-and-eosin techniques.

RESULTS

Paclitaxel concentrations (maximum concentration and area under the plasma concentration time curve) in perfused tissues increased with escalating perfusate doses. Uptake of drug into lung parenchyma appeared saturable at high paclitaxel exposure; a substantial pharmacokinetic advantage was observed. Paclitaxel concentrations in systemic circulation were undetectable or exceedingly low after perfusion. Histopathologic examination of lung tissues harvested 3 hours after completion of isolated lung perfusion revealed no immediate toxicity, even at a paclitaxel exposure 20-fold higher than that achievable after 1 hour of intravenous administration at the maximum tolerable dose in human subjects. Moderate hyperthermia enhanced paclitaxel-mediated cytotoxicity 5- to 100-fold in cultured cancer lines. No paclitaxel toxicity was observed in cultured normal human bronchial epithelial cells after exposure to paclitaxel under normothermic or hyperthermic conditions.

CONCLUSIONS

These data support further evaluation of paclitaxel administered by hyperthermic retrograde isolated lung perfusion techniques for the treatment of unresectable malignant pulmonary tumors.

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.