Biomedical subjects

Andrew D Levin

Publications and source records attributed to Andrew D Levin.

4 recordsLinked to original sources

PubMed2005

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.

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PubMed2005

Thrombosis modulates arterial drug distribution for drug-eluting stents.

BACKGROUND: Drug-eluting stents deliver potent compounds directly to arterial segments but can become clot laden when deployed. The question arises as to whether thrombi affect drug elution and arterial uptake. METHODS AND RESULTS: Paclitaxel transport and retention were assessed in clots of different blood components. Diffusivity, affected by clot organization, is fastest in fibrin (approximately 347 microm2/s), slower in fibrin-red blood cell clots (34.98 microm2/s), and slowest in whole-blood clots (3.55 microm2/s). Blood cells bind and retain paclitaxel such that levels in clot increase linearly with red cell fraction. At physiological hematocrit, clot retains 3 times the amount of paclitaxel in surrounding solutions. Computational models predict that the potential of thrombus to absorb, retain, and release drug or to act as a barrier to drug delivery depends on clot geometry and strut position in clot relative to the vessel wall. Clot between artery and stent can reduce uptake 10-fold, whereas clot overlying the stent can shield drug from washout, increasing uptake. Model assumptions were confirmed and predictions were validated in a novel rat model that introduces thrombosis within stented aortas where nonocclusive thrombus acts as capacitive space for drug and shifts drug levels to decrease tissue uptake 2-fold. CONCLUSIONS: Thrombus apposed on stents creates large variations in drug uptake and can act to either increase or decrease wall deposition according to the clot and stent geometry. Arterial deposition of drug from stents deployed in clots will be highly variable and unpredictable unless the clot can be adequately controlled or removed.

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PubMed2004

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.

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PubMed2002

Phosphorylation-induced conformational changes in a mitogen-activated protein kinase substrate. Implications for tyrosine hydroxylase activation.

Mitogen-activated protein (MAP) kinase-mediated phosphorylation of specific residues in tyrosine hydroxylase leads to an increase in enzyme activity. However, the mechanism whereby phosphorylation affects enzyme turnover is not well understood. We used a combination of fluorescence resonance energy transfer (FRET) measurements and molecular dynamics simulations to explore the conformational free energy landscape of a 10-residue MAP kinase substrate found near the N terminus of the enzyme. This region is believed to be part of an autoregulatory sequence that overlies the active site of the enzyme. FRET was used to measure the effect of phosphorylation on the ensemble of peptide conformations, and molecular dynamics simulations generated free energy profiles for both the unphosphorylated and phosphorylated peptides. We demonstrate how FRET transfer efficiencies can be calculated from molecular dynamics simulations. For both the unphosphorylated and phosphorylated peptides, the calculated FRET efficiencies are in excellent agreement with the experimentally determined values. Moreover, the FRET measurements and molecular simulations suggest that phosphorylation causes the peptide backbone to change direction and fold into a compact structure relative to the unphosphorylated state. These results are consistent with a model of enzyme activation where phosphorylation of the MAP kinase substrate causes the N-terminal region to adopt a compact structure away from the active site. The methods we employ provide a general framework for analyzing the accessible conformational states of peptides and small molecules. Therefore, they are expected to be applicable to a variety of different systems.

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