Recommendations on the Use of Multiple Labels in Human Mass Balance Studies.

The administration of radiolabeled drug candidates is considered the gold standard in absorption, distribution, metabolism, and excretion studies for small-molecule drugs since it allows facile and accurate quantification of parent drug, metabolites, and total drug-related material independent of the compound structure. The choice of the position of the radiolabel, typically 14C or 3H, is critical to obtain relevant information. Sometimes, a biotransformation reaction may lead to cleavage of a part of the molecule. As a result, only the radiolabeled portion can be followed, and information on the fate of the nonlabeled metabolite may be lost. Synthesis and administration of two or more radiolabeled versions of the parent drug as a mixture or in separate studies may resolve this issue but comes with additional challenges. In this paper, we address the questions that may be considered to help make the right choice whether to use a single or multiple radiolabel approach and discuss the pros and cons of different multiple-labeling strategies that can be taken as well as alternative methods that allow the nonlabeled part of the molecule to be followed. SIGNIFICANCE STATEMENT: Radiolabeled studies are the gold standard in drug metabolism research, but molecules can undergo cleavage with loss of the label. This often results in discussions around potential use of multiple labels, which seem to be occurring with increased frequency since an increasing proportion of the small-molecule drugs are tending towards larger molecular weights. This review provides insight and decision criteria in considering a multiple-label approach as well as pros and cons of different strategies that can be followed.

Study of the Distribution of Acetaminophen and Its Metabolites in Rats, from the Whole-Body to Isolated Organ Levels, by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging after On-Tissue Chemical Derivatization.

During drug development, detailed investigations of the pharmacokinetic profile of the drug are required to characterize its absorption, distribution, metabolism, and excretion properties. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) is an established technique for studies of the distribution of drugs and their metabolites. It has advantages over autoradiography, which is conventionally used for distribution studies: it does not require the radiolabeling of drugs and can distinguish between the drug and its metabolites directly in the tissue. However, its lack of sensitivity in certain cases remains challenging. Novel procedures, such as on-tissue chemical derivatization (OTCD), could be developed to increase sensitivity. We used OTCD to enhance the sensitivity of MALDI-MSI for one of the most widely used drugs, acetaminophen, and to study its distribution in tissues. Without derivatization, this drug and some of its metabolites are undetectable by MALDI-MSI in the tissues of treated rats. We used 2-fluoro-1-methylpyridinium p-toluene sulfonate as a derivatization reagent, to increase the ionization yield of acetaminophen and some of its metabolites. The OTCD protocol made it possible to study the distribution of acetaminophen and its metabolites in whole-body sections at a spatial resolution of 400 µm and in complex anatomical structures, such as the testis and epididymis, at a spatial resolution <50 µm. The OTCD is also shown to be compatible with the quantification of acetaminophen by MALDI-MSI in whole-body tissues. This protocol could be applied to other molecules bearing phenol groups and presenting a low ionization efficiency.

Non-Labeled, Stable Labeled, or Radiolabelled Approaches for Provision of Intravenous Pharmacokinetics in Humans: A Discussion Piece.

A review of the use of microdoses and isotopic microtracers for clinical intravenous pharmacokinetic (i.v. PK) data provision is presented. The extent of application of the varied approaches available and the relative merits of each are highlighted with the aim of assisting practitioners in making informed decisions on the most scientifically appropriate design to adopt for any given new drug in development. It is envisaged that significant efficiencies will be realized as i.v. PK data in humans becomes more routinely available for suitable assets in early development, than has been the case prior to the last decade.

Considerations for Human ADME Strategy and Design Paradigm Shift(s) - An Industry White Paper.

The human absorption, distribution, metabolism, and excretion (hADME) study is the cornerstone of the clinical pharmacology package for small molecule drugs, providing comprehensive information on the rates and routes of disposition and elimination of drug-related material in humans through the use of 14 C-labeled drug. Significant changes have already been made in the design of the hADME study for many companies, but opportunity exists to continue to re-think both the design and timing of the hADME study in light of the potential offered by newer technologies, that enable flexibility in particular to reducing the magnitude of the radioactive dose used. This paper provides considerations on the variety of current strategies that exist across a number of pharmaceutical companies and on some of the ongoing debates around a potential move to the so called "human first/human only" approach, already adopted by at least one company. The paper also provides a framework for continuing the discussion in the application of further shifts in the paradigm.

On-tissue chemical derivatization reagents for matrix-assisted laser desorption/ionization mass spectrometry imaging.

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) is a key tool for the analysis of biological tissues. It provides spatial and quantitative information about different types of analytes within tissue sections. Despite the increasing improvements of this technique, the low detection sensitivity of some compounds remains an important challenge to overcome. Poor sensitivity is related to weak ionization efficiency, low abundance of analytes and matrix ions, or endogenous interferences. On-tissue chemical derivatization (OTCD) has proven to be an important solution to these issues and is increasingly employed in MALDI MSI studies. OTCD reagents, synthesized or commercially available, have been essentially used for the detection of small exogenous or endogenous molecules within tissues. Optimally, an OTCD reaction is performed in mild conditions, in an acceptable range of time, preserves the integrity of the tissues, and prevents the delocalization. In addition to their reactivity with a targeted chemical function, some OTCD reagents can also be used as a matrix, which simplifies the sample preparation procedure. In this review, we present an exhaustive overview of OTCD reagents and methods used in MALDI MSI studies.

The mitochondrial fluorescent dye rhodamine 123 is a high-affinity substrate for organic cation transporters (OCTs) 1 and 2.

Rhodamine 123 is a fluorescent cationic dye commonly used as a mitochondrial probe and known or suspected to be transported by certain drug membrane transporters. The present study was designed to characterize the putative interactions of rhodamine 123 with human organic cation transporter (OCT) 1 and OCT2. Intracellular uptake of the dye was demonstrated to be enhanced in both hOCT1- and hOCT2-overexpressing HEK293 cells when compared with control HEK293 cells. This increase of rhodamine 123 influxes was found to be a saturable carrier-mediated process, with low K(m) values (K(m) = 0.54 µm and K(m) = 0.61 µm for transport of the dye in hOCT1- and hOCT2-positive HEK293 cells, respectively). Known inhibitors of hOCT1 and hOCT2 activities such as verapamil, amitriptyline, prazosin, and quinine were next demonstrated to decrease rhodamine 123 accumulation in hOCT1- and hOCT2-overexpressing HEK293 cells. In addition, the dye was found to inhibit hOCT1- and hOCT2-mediated uptake of tetraethylammonium (TEA), a model substrate for both hOCT1 and hOCT2; rhodamine 123 appeared nevertheless to be a more potent inhibitor of hOCT1-mediated TEA transport (IC50 = 0.37 µm) than of that mediated by hOCT2 (IC50 = 61.5 µm). Taken together, these data demonstrate that rhodamine 123 is a high-affinity substrate for both hOCT1 and hOCT2. This dye may be therefore useful for fluorimetrically investigating cellular hOCT1 or hOCT2 activity, knowing, however, that other factors potentially contributing to cellular accumulation of rhodamine 123, including mitochondrial membrane potential or expression of the efflux transporter P-glycoprotein, have also to be considered.

S49076 is a novel kinase inhibitor of MET, AXL, and FGFR with strong preclinical activity alone and in association with bevacizumab.

Aberrant activity of the receptor tyrosine kinases MET, AXL, and FGFR1/2/3 has been associated with tumor progression in a wide variety of human malignancies, notably in instances of primary or acquired resistance to existing or emerging anticancer therapies. This study describes the preclinical characterization of S49076, a novel, potent inhibitor of MET, AXL/MER, and FGFR1/2/3. S49076 potently blocked cellular phosphorylation of MET, AXL, and FGFRs and inhibited downstream signaling in vitro and in vivo. In cell models, S49076 inhibited the proliferation of MET- and FGFR2-dependent gastric cancer cells, blocked MET-driven migration of lung carcinoma cells, and inhibited colony formation of hepatocarcinoma cells expressing FGFR1/2 and AXL. In tumor xenograft models, a good pharmacokinetic/pharmacodynamic relationship for MET and FGFR2 inhibition following oral administration of S49076 was established and correlated well with impact on tumor growth. MET, AXL, and the FGFRs have all been implicated in resistance to VEGF/VEGFR inhibitors such as bevacizumab. Accordingly, combination of S49076 with bevacizumab in colon carcinoma xenograft models led to near total inhibition of tumor growth. Moreover, S49076 alone caused tumor growth arrest in bevacizumab-resistant tumors. On the basis of these preclinical studies showing a favorable and novel pharmacologic profile of S49076, a phase I study is currently underway in patients with advanced solid tumors. Mol Cancer Ther; 12(9); 1749-62. ©2013 AACR.

Stimulation of tumor growth and angiogenesis by low concentrations of RGD-mimetic integrin inhibitors.

Inhibitors of alpha(v)beta(3) and alpha(v)beta(5) integrin have entered clinical trials as antiangiogenic agents for cancer treatment but generally have been unsuccessful. Here we present in vivo evidence that low (nanomolar) concentrations of RGD-mimetic alpha(v)beta(3) and alpha(v)beta(5) inhibitors can paradoxically stimulate tumor growth and tumor angiogenesis. We show that low concentrations of these inhibitors promote VEGF-mediated angiogenesis by altering alpha(v)beta(3) integrin and vascular endothelial growth factor receptor-2 trafficking, thereby promoting endothelial cell migration to VEGF. The proangiogenic effects of low concentrations of RGD-mimetic integrin inhibitors could compromise their efficacy as anticancer agents and have major implications for the use of RGD-mimetic compounds in humans.

Evaluation of the cytotoxicity effect of dimethyl sulfoxide (DMSO) on Caco2/TC7 colon tumor cell cultures.

Dimethyl sulfoxide (DMSO) is usually used to solubilize poorly soluble drugs in permeation assays such as that using Caco2 enterocyte-like cells. The objective of this study was to evaluate the toxicity of DMSO on Caco2/TC7 cells and determinate the maximal concentration usable in permeation experiments. Caco2/TC7 cells were cultured for 21 d on 96-well plates for evaluation of toxicity. The determination of lactate dehydrogenase (LDH) release in cell supernatant and the measurement of Neutral Red (NR) uptake are used for cytotoxicity assays. DMSO solutions (0-100%) in Hank's balanced salt solution containing HEPES (25 mM), pH 7.4, were incubated with Caco-2/TC7 cells on 96 well plates. Caco2/TC7 cells were cultured on Transwell-Clear inserts to evaluate the influence of DMSO on the apparent permeability of the paracellular marker mannitol. DMSO 10% did not induce any significant increase in LDH release whereas a significant increase in LDH activity (ANOVA, p<0.05) occurred at a DMSO concentration of 20 to 50%. NR incorporation in viable cells was statistically reduced by 27 to 36% at DMSO concentration of 20% up to 100% (ANOVA, p>0.05). No statistical difference (p<0.05) in apparent mannitol permeability was observed between the control and 10% DMSO groups. In conclusion, at concentrations of up to 10%, DMSO did not produce any significant alteration in apical membrane permeability or on cell-to-cell tight junctional complexes.

Short term Caco-2/TC7 cell culture: comparison between conventional 21-d and a commercially available 3-d system.

The Caco-2 cell model is a valuable tool for studying intestinal biotransformation of xenobiotics and to evaluate the potential of human intestinal absorption of new compounds. These properties were evaluated with Caco-2/TC7 cells in accelerated conditions to reduce maturation lag time from 21-d to 3-d in order to increase time and labor efficiency. Transmission electron and fluorescent microscopy were used for morphological characterization. Alkaline phosphatase and lactate dehydrogenase activities were assessed within time. Cytochrome P450 expression was studied by RT-PCR. Apparent permeabilities of a set of passively absorbed molecules across Caco-2/TC7 cell monolayers were determined to evaluate potential of both systems for prediction of human intestinal absorption. Microscopic images revealed that cells under both conditions differentiated as enterocyte-like cells but did so heterogeneously in the 3-d model. TEER values have shown that the 3-d model is a leakier cell system with higher mannitol Papp (cm/s). Biochemical characterization (hydrolase activities, CYP450 expression) suggested that the 3-d model was at a lower maturation level than the 21-d model. Carrier-mediated uptake of L-Phe was lower in the 3-d model suggesting that this model has limited application for mechanistic studies. Reasonable correlation was obtained between the two models (r2=0.88, p>0.01) for 11 passively absorbed compounds with high potential of rank ordering of compounds. Although results suggested that the 3-d cells are under-differentiated, they could be usable to estimate the oral absorption of passively absorbed compounds.