RESEARCH & DEVELOPMENT

Novel local delivery strategies for preventing restenosis

-JoBames-

Local delivery of biologically active agents is a promising approach to preventing restenosis, according to data presented at the 17th Congress of the European Society of Cardiology [Amsterdam, The Nether/Qnds; August 1995]. us researchers aim to improve upon existing loc::al delivery techniques by using biodegradable polymeric nanoparticles as a carrier for drugs wbich prevent neoiotimal proliferation. Other researchers have developed a fibrin adhesive capable of attachjng endothelial cells to damaged arterial walls. Findings from preclinical studies with these strategies and their potential for clinical application were discussed at the meeting.

Local delivery of agents to prevent restenosis has advantages over systemic treatments, although local drug delivery itself is not without problems. Vascular injury, low efficiency of drug delivery. tissue incom­patibility and significant systemic effects are some of the problems encountered with currently available local drug delivery systems, according to Dr L Guzman from the Cleveland Clinic Foundation, Cleveland, US.

In an. attempt to overcome some of these limitations, Dr Guzman and colleagues have developed biodegradable polymeric nanoparticles constructed from polylactic glycolic acid (PLGA) as a carrier system for local delivery of agents to prevent restenosis. L

PLGA nanoparticles deliver tile goods Dr Guzman listed several characteristics that are

required in order for a carrier to be effective as a local delivery system [see boxed text). Referring to PLGA nanoparticles, Dr Guzman said, 'we have developed a carrier that meets most of these conditions'.

'" Ideal c:haraco1erisI! of CI!II'Ier ~ for local drug delivery to pmvent I1!SIenoBis "

• Capacity for loading with large amounts of therapeutic agents '"

• Slow and sustained release of drugs • Biocompmible • High entrapment efficiency • Biodegradable • Smail particle size permitting use of currently

available balloon catheter delivery systems

IJexamethasone.loaded PLGAnanoparIides reduce neointimal fonnatioo

Dr Guzman's group used PLGA nanopanicles matrixed with dexamethasone to evaluate their effects on neointimal formation in Sprague·Dawley rats with balloon angioplasty·injured carotid arteries. Summarising the results. Dr Guzman said, 'dexamethasone-loaded nanoparticles, locally infused, markedly reduced neointimal formation in this animal model'.

In the study, rats received a single 3·minute intra· luminal infusion of IOmg of dexamethasone-loaded PLGA nanoparticles in saline into the carotid anery. Dexamethasone was used on the basis of data from previous studies which showed that this agent signifi­cantly inhibits neointimal proliferation in animal models.

Control groups received either saline alone, or an intraperitoneal infusion of 10mg of dexamethasone­loaded PLGA nanoparticles in saline. The latter group was included to determine whether a systemic effect

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could be responsible for the study findings. AnimaJs were sacrificed 3 weeks after infusion of PLGA nano· particles and saJine.

A significant increase in the lumen area was seen in the aneries of rats treated with intraluminal dexa· methasone-loaded PLGA nanoparticles, when compared with those in control rats (p = 0.01). In addition, there was a 33% reduction in the intima:media ratio in the treatment group, compared with those in the control groups.

SiU><peciIic: delivery achieved Analysis of tissue segmems taken from rats treated

with locally infused dexamethasone· loaded PLGA nano­particles revealed that the concentration of dexamethasone in treated arterial segments 3 hours postinfusion was 95·times higher than that in contralateral segments. According to Dr Guzman, hig h dexamethasone con· centrations at the delivery site persisted for more than 7 days.

'Biodegradable nanoparticles may represent a new modality for site-specific drug delivery, and have clearcut poltmtial

Jor clinical application.' Dr Luis Guzman. The Clfiltdand Clinic Foundmion,

CkvelmuJ, US

Other investigations with fluorescent·labelled. unmatrixed PLGA nanoparticles demonstrated pene­tration of the nanoparticles into the intimal, mediaJ and adventitial layers of balloon angioplasty·injured rat carotid arteries. After 3 days, no significant fluorescent activity was detected in the intimaJ or medial layers of the vessel wall, aJthough significant fluorescent activity was still present in the adventitial layer of the anery for 7 days.

AdventiliaJ layer - a potential target? SeveraJ studies have suggested that the adventitial

layer may playa major role in the process of restenosis, according to Dr Guzman. He suggested that by targeting the adventitia it may be possible to prevent the develop. ment of neointiOlaJ proliferation. 'The adventitia is an area that we are paying more attention to', he said. 'Whether there is diffusion through the adventitia to the intima has not been proved, but it might be that the adventitia acts as a reservoir for the drug', he added.

When questioned whether PLGA nanoparticles would be effective as a carrier in atherosclerotic aneries, Dr Guzman replied, 'Yes . .. I would imagine that the penetration is going to be greater because there is more disruption of the different layers of the arterial wall'. He revealed that his group are currently

9

10 RESEARCH & DEVELOPMENT

Local delivery strategies for preventing restenosis - continued

investigating the effects of biodegradable polymeric nanoparticles in a rabbit model of atherosclerosis.

I'ndotboJial cell denudation pivotal in_ Dr J Baker from the University of Wisconsin Medical

School and colleagues from the Medical College of Wisconsin, Milwaukee, US, proposed that denudation of the endothelial celiliniog of arterial walls during percutaneous translumina1 coronary angioplasty (PTCA) may be a pivotal event in the restenotic process. Thus, 'the establishment of a viable and confluent endo­thelial cell Layer in damaged arteries may mitigate restenosis', Dr Baker suggested. Dr Baker's group has now developed a fibrin-based adhesive that can attach endothelial cells to the arterial walls of animals with balloon angioplasty-injured arteries [see boxed text].

Fonnation of fibrin acIlesIva'endoI cell matrix

Fibrinogen and certain other components necessary for preparation of the 'biological glue' are obtained from autologous blood by centrifugation and cryopreservation. Autologous endothelial cells are obtained by liposuction, and then purified and cultured to achieve ,a large population of cells. Adding the serum protease thrombin to the fibrinogen and endothelial cell mixture initiates the formation of a fibrin adhesive/endothelial cell matrix. Thrombin cleaves fibrinogen and produces fibrin strands which will anchor the endothelial cells to the arterial surface. The fibrin adhesive/endothelial cell matrix is delivered locally to the damaged artery via a balloon catheter.

FThrin adhesive attaches endoCheliaI cells A study in New Zealand White rabbits with balloon

angioplasty-induced endothelial cell-denuded iliac arteries demonstrated that fibrin adhesive can attach endothelial cells to damaged arterial surfaces. said Dr Baker. 'We are getting a carpel of {endothelial] cells' , be added.

Animals received fluorescent -stained fibrin adhesive/endothelial cell matrix, or endothelial cells alone. via a balloon catheter over 3-5 minutes. Three hours after treatment. histological examinations revealed approximately 80% circumferential reseeding with endothelial cells in the arteries of animals that received the fibrin adhesive/endothelial cell matrix. In contrast, only about 8% circumferential reseeding was seen in the arteries of control animals.

Limi1ations oot insunnoontable Clearly this work is still in the early stages and

several limitations must be overcome before the tech­nique can be considered for clinical application.

In particular, endothelial cells need to be cultured for several days to obtain a large enough population for performing the technique. Dr Baker suggested that in the clinical arena, one approach to overcoming this may be to obtain autologous endothelial cells before PTCA is performed.

Long-term preclinical studies are now underway and 'if we are successful, then we can begin to talk more about the possibility of using this approach clinically', Dr Baker said. 1. GUZllWl LA. e\ Ii. Local intraluminal infusion of biodegradable poIymcri<: .. anopa:tidt:$ matrixcd with dcumetbasone. A DOVd antiproliferatiye approa.:h for prolollgcd Mit deliyery afu:r balloon IlIgioplasty. European Heart J(ltIJ1IaI 16 (Suppl.): 424. Aug 1995 OODl, ....

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