New Insights in Tissue Distribution, Metabolism, and Excretion of [3H]-Labeled Antibody Maytansinoid Conjugates in Female Tumor-Bearing Nude Rats.

For antibody drug conjugates (ADCs), the fate of the cytotoxic payload in vivo needs to be well understood to mitigate toxicity risks and properly design the first in-patient studies. Therefore, a distribution, metabolism, and excretion (DME) study with a radiolabeled rat cross-reactive ADC ([(3)H]DM1-LNL897) targeting the P-cadherin receptor was conducted in female tumor-bearing nude rats. Although multiple components [total radioactivity, conjugated ADC, total ADC, emtansine (DM1) payload, and catabolites] needed to be monitored with different technologies (liquid scintillation counting, liquid chromatography/mass spectrometry, enzyme-linked immunosorbent assay, and size exclusion chromatography), the pharmacokinetic data were nearly superimposable with the various techniques. [(3)H]DM1-LNL897 was cleared with half-lives of 51-62 hours and LNL897-related radioactivity showed a minor extent of tissue distribution. The highest tissue concentrations of [(3)H]DM1-LNL897-related radioactivity were measured in tumor. Complimentary liquid extraction surface analysis coupled to micro-liquid chromatography-tandem mass spectrometry data proved that the lysine (LYS)-4(maleimidylmethyl) cyclohexane-1-carboxylate-DM1 (LYS-MCC-DM1) catabolite was the only detectable component distributed evenly in the tumor and liver tissue. The mass balance was complete with up to 13.8% ± 0.482% of the administered radioactivity remaining in carcass 168 hours postdose. LNL897-derived radioactivity was mainly excreted via feces (84.5% ± 3.12%) and through urine only to a minor extent (4.15% ± 0.462%). In serum, the major part of radioactivity could be attributed to ADC, while small molecule disposition products were the predominant species in excreta. We show that there is a difference in metabolite profiles depending on which derivatization methods for DM1 were applied. Besides previously published results on LYS-MCC-DM1 and MCC-DM1, maysine and a cysteine conjugate of DM1 could be identified in serum and excreta.

Binding of pimecrolimus and tacrolimus to skin and plasma proteins: implications for systemic exposure after topical application.

Pimecrolimus and tacrolimus are calcineurin inhibitors used for the topical treatment of atopic dermatitis. Although structurally similar, they display specific differences including higher lipophilicity and lower skin permeation of pimecrolimus. The aim of the present study was to understand the reason for the differences in skin permeation; in addition, plasma protein binding of the two drugs was analyzed side by side as a basis for comparison of systemic exposure to free drug. Permeation of pimecrolimus and tacrolimus through a silicon membrane was found to be similar; therefore, we assumed that differences in skin permeation could be caused by differences in affinity to skin components. To test this hypothesis, we investigated binding of pimecrolimus and tacrolimus to a preparation of soluble human skin proteins. One binding protein of approximately 15 kDa, probably corresponding to macrophilin12, displayed a similar binding capacity for pimecrolimus and tacrolimus. However, less specific, nonsaturating binding to other proteins was approximately 3-fold higher for pimecrolimus. Because of the high local drug concentration after topical administration, the unspecific, high-capacity binding is probably dominating the permeation through skin. In plasma both drugs bound predominantly to lipoproteins, which may affect disposition differently from albumin binding. The unbound fraction of pimecrolimus in human plasma was approximately 9-fold lower compared with that of tacrolimus (0.4 +/- 0.1 versus 3.7 +/- 0.8%). In conclusion, these results provide an explanation for the observed lower systemic exposure to pimecrolimus than to tacrolimus after topical application and suggest that differences in systemic exposure to free drug might be even more pronounced.

ABP688, a novel selective and high affinity ligand for the labeling of mGlu5 receptors: identification, in vitro pharmacology, pharmacokinetic and biodistribution studies.

[(11)C]ABP688 (2) has recently been demonstrated to be a useful PET tracer for in vivo imaging of the metabotropic glutamate receptors type 5 (mGluR5) in rodents. We describe here the identification and preclinical profiling of ABP688 and its tritiated version [(3)H]ABP688, and show that its high affinity (K(d)=2nM), selectivity, and pharmacokinetic properties fulfill all requirements for development as a PET tracer for clinical imaging of the mGlu5 receptor.

Pimecrolimus: absorption, distribution, metabolism, and excretion in healthy volunteers after a single oral dose and supplementary investigations in vitro.

The absorption and disposition of pimecrolimus, a calcineurin inhibitor developed for the treatment of inflammatory skin diseases, was investigated in four healthy volunteers after a single oral dose of 15 mg of [(3)H]pimecrolimus. Supplementary information was obtained from in vitro experiments. Pimecrolimus was rapidly absorbed. After t(max) (1-3 h), its blood concentrations fell quickly to 3% of C(max) at 24 h, followed by a slow terminal elimination phase (average t(1/2) 62 h). Radioactivity in blood decreased more slowly (8% of C(max) at 24 h). The tissue and blood cell distribution of pimecrolimus was high. The metabolism of pimecrolimus in vivo, which could be well reproduced in vitro (human liver microsomes), was highly complex and involved multiple oxidative O-demethylations and hydroxylations. In blood, pimecrolimus was the major radiolabeled component up to 24 h (49% of radioactivity area under the concentration-time curve(0-24) h), accompanied by a large number of minor metabolites. The average fecal excretion of radioactivity between 0 and 240 h amounted to 78% of dose and represented predominantly a complex mixture of metabolites. In urine, 0 to 240 h, only about 2.5% of the dose and no parent drug was excreted. Hence, pimecrolimus was eliminated almost exclusively by oxidative metabolism. The biotransformation of pimecrolimus was largely catalyzed by CYP3A4/5. Metabolite pools generated in vitro showed low activity in a calcineurin-dependent T-cell activation assay. Hence, metabolites do not seem to contribute significantly to the pharmacological activity of pimecrolimus.