The pH-selective insertion and folding of a membrane peptide, pHLIP [pH (low) insertion peptide], can be used to target acidic tissue in vivo, including acidic foci in tumors, kidneys, and inflammatory sites. In a mouse breast adenocarcinoma model, fluorescently labeled pHLIP finds solid acidic tumors with high accuracy and accumulates in them even at a very early stage of tumor development. The fluorescence signal is stable for >4 days and is approximately five times higher in tumors than in healthy counterpart tissue. In a rat antigen-induced arthritis model, pHLIP preferentially accumulates in inflammatory foci. pHLIP also maps the renal cortical interstitium; however, kidney accumulation can be reduced significantly by providing mice with bicarbonatecontaining drinking water. The peptide has three states: soluble in water, bound to the surface of a membrane, and inserted across the membrane as an ␣-helix. At physiological pH, the equilibrium is toward water, which explains its low affinity for cells in healthy tissue; at acidic pH, titration of Asp residues shifts the equilibrium toward membrane insertion and tissue accumulation. The replacement of two key Asp residues located in the transmembrane part of pHLIP by Lys or Asn led to the loss of pH-sensitive insertion into membranes of liposomes, red blood cells, and cancer cells in vivo, as well as to the loss of specific accumulation in tumors. pHLIP nanotechnology introduces a new method of detecting, targeting, and possibly treating acidic diseased tissue by using the selective insertion and folding of membrane peptides.cancer targeting ͉ imaging ͉ peptide insertion M any pathological conditions such as cancer, ischemic stroke, inflammation, atherosclerotic plaques, and others are associated with increased metabolic activity and hypoxia resulting in an elevated extracellular acidity (1-6). Hypoxia and acidity have emerged as important factors in tumor biology and responses to cancer treatment. Imaging of hypoxic and acidic regions could provide new information about disease localization and progression and might enhance diagnosis and therapy. Here we describe the use of a peptide, pHLIP [pH (low) insertion peptide], to selectively accumulate in and label acidic tissues. We previously reported that the pHLIP bionanosyringe, a 36-aa peptide derived from the bacteriorhodopsin C helix, has three states: soluble in water, bound to the surface of a membrane, and inserted across the membrane as an ␣-helix. At physiological pH the water-soluble form is favored, whereas at acidic pH the transmembrane ␣-helix predominates (Fig. 1a) (7). We show that at low pH, pHLIP can translocate cell-impermeable cargo molecules that are disulfide-linked to the C terminus across a cell membrane, where they are released in the cytoplasm by reduction of the disulfide bond (8, 9). Among the successfully translocated molecules are organic dyes, phalloidin (a polar, cyclic peptide), and a 12-mer peptide nucleic acid.

Given the proven utility of natriuretic peptides as serum biomarkers of cardiovascular maladaptation and dysfunction in humans and the high cross-species sequence conservation of atrial natriuretic peptides, natriuretic peptides have the potential to serve as translational biomarkers for the identification of cardiotoxic compounds during multiple phases of drug development. This work evaluated and compared the response of N-terminal proatrial natriuretic peptide (NT-proANP) and N-terminal probrain natriuretic peptide (NT-proBNP) in rats during exercise-induced and drug-induced increases in cardiac mass after chronic swimming or daily oral dosing with a peroxisome proliferator-activated receptor γ agonist. Male Sprague-Dawley rats aged 8 to 10 weeks were assigned to control, active control, swimming, or drug-induced cardiac hypertrophy groups. While the relative heart weights from both the swimming and drug-induced cardiac hypertrophy groups were increased 15% after 28 days of dosing, the serum NT-proANP and NT-proBNP values were only increased in association with cardiac hypertrophy caused by compound administration. Serum natriuretic peptide concentrations did not change in response to adaptive physiologic cardiac hypertrophy induced by a 28-day swimming protocol. These data support the use of natriuretic peptides as fluid biomarkers for the distinction between physiological and drug-induced cardiac hypertrophy.

Evidence from our laboratory suggests that tolfenamic acid has a potential for slowing the progression of Alzheimer's disease (AD) through lowering cortical levels of the β-amyloid precursor protein (APP) and its pathogenic amyloid beta (Aβ) intermediates [1]. In this study, we examined the ability of tolfenamic acid to cross the blood brain barrier (BBB) by predicting its logBB and logPS values, the indexes of BBB permeability, using computational models. We also determined, via in vitro methods, the brain penetration capacity factor [(K(IAM)/MW(4))x10(10)] using phosphatidylcholine column chromatography. The obtained logBB, logPS and (K(IAM)/MW(4))x10(10) values predicted that tolfenamic acid can passively transfer into the central nervous system (CNS). These results were validated in vivo using LC-MS analysis after administration of tolfenamic acid intravenously to guinea pigs and mice. The present study provides the first evidence of the ability of tolfenamic acid to cross the BBB and offers a comparative analysis of approaches used to predict the ability of compounds to penetrate into the brain.

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