Mechanism of deiodination of 125I-human growth hormone in vivo. Relevance to the study of protein disposition

Plasma half-life and tissue distribution of leukocyte cell-derived chemotaxin 2 in mice

Leukocyte cell-derived chemotaxin 2 (LECT2) is a hepatokine that causes skeletal muscle insulin resistance. The circulating levels of LECT2 are a possible biomarker that can predict weight cycling because they reflect liver fat and precede the onset of weight loss or gain. Herein, to clarify the dynamics of this rapid change in serum LECT2 levels, we investigated the in vivo kinetics of LECT2, including its plasma half-life and tissue distribution, by injecting ¹²⁵I-labelled LECT2 into ICR mice and radioactivity tracing. The injected LECT2 was eliminated from the bloodstream within 10 min (approximate half-life, 5 min). In the kidneys, the radioactivity accumulated within 10 min after injection and declined thereafter. Conversely, the radioactivity in urine increased after 30 min of injection, indicating that LECT2 is mainly excreted by the kidneys into the urine. Finally, LECT2 accumulated in the skeletal muscle and liver until 30 min and 2 min after injection, respectively. LECT2 accumulation was not observed in the adipose tissue. These findings are in agreement with LECT2 action on the skeletal muscle. The present study indicates that LECT2 is a rapid-turnover protein, which renders the circulating level of LECT2 a useful rapid-response biomarker to predict body weight alterations.

Relevance of radiobiological concepts in radionuclide therapy of cancer

PURPOSE

Radionuclide therapy (RNT) is a rapidly growing area of clinical nuclear medicine, wherein radionuclides are employed to deliver cytotoxic dose of radiation to the diseased cells/tissues. During RNT, radionuclides are either directly administered or delivered through biomolecules targeting the diseased site. RNT has been clinically used for diverse range of diseases including cancer, which is the focus of the review.

CONCLUSIONS

The major emphasis in RNT has so far been given towards developing peptides/antibodies and other molecules to conjugate a variety of therapeutic radioisotopes for improved targeting/delivery of radiation dose to the tumor cells. Despite that, many of the RNT approaches have not achieved their desired therapeutic success probably due to poor knowledge about complex and dynamic (i) fate of radiolabeled molecules; (ii) radiation dose delivered; (iii) cellular heterogeneity in tumor mass; and (iv) cellular radiobiological response. Based on understanding gathered during recent years, it may be stated that besides the absorbed dose, the net radiobiological response of tumor/normal cells also determines the clinical response of radiotherapeutic modalities including RNT. The radiosensitivity of tumor/normal cells is governed by radiobiological phenomenon such as radiation-induced bystander effect, genomic instability, adaptive response and low dose hyper-radiosensitivity. These concepts have been well investigated in the context of external beam radiotherapy, but their clinical implications during RNT have received meagre attention. In this direction, a few studies performed using in vitro and in vivo models envisage the possibilities of exploiting the radiobiological knowledge for improved therapeutic outcome of RNT.

Serum stability of peptides

Hospitals worldwide have lately reported a worrying increase in the number of isolated drug-resistant pathogenic microbes. This has to some extent fueled at least academic interest in design and development of new lead components for novel drug design. Much of this interest has been focused on antimicrobial peptides and peptides in general, primarily due to their natural occurrence and low toxicity. However, issues have been raised regarding the stability of peptide therapeutics for systemic use. The focus of this chapter is assays for measuring peptide stability in the presence of serum, both in vitro and in vivo.

Methods for Predicting Human Drug Metabolism

Drug metabolism information is a necessary component of drug discovery and development. The key issues in drug metabolism include identifying: the enzyme(s) involved, the site(s) of metabolism, the resulting metabolite(s), and the rate of metabolism. Methods for predicting human drug metabolism from in vitro and computational methodologies and determining relationships between the structure and metabolic activity of molecules are also critically important for understanding potential drug interactions and toxicity. There are numerous experimental and computational approaches that have been developed in order to predict human metabolism which have their own limitations. It is apparent that few of the computational tools for metabolism prediction alone provide the major integrated functions needed to assist in drug discovery. Similarly the different in vitro methods for human drug metabolism themselves have implicit limitations. The utilization of these methods for pharmaceutical and other applications as well as their integration is discussed as it is likely that hybrid methods will provide the most success.

Tracking the in vivo fate of recombinant polypeptides by isotopic labeling

We report a method to incorporate a stable isotope (13C) and a radioactive isotope (14C) into a recombinant polypeptide during Escherichia coli culture in M9 minimal medium supplemented with universally labeled 13C- or 14C-labeled glucose. We chose a thermally responsive elastin-like polypeptide (ELP) as a model polypeptide for this study because of its utility in various biotechnology applications such as drug delivery and tissue engineering. High cell densities were obtained by step-wise adaptation of E. coli to M9 medium in addition to supplementing the medium with trace elements that facilitated growth of E. coli. Furthermore, an optimal concentration of isopropyl-beta-d-thiogalactopyranoside was determined for induction of ELP expression to achieve high yield (mg/L culture) of the ELP. The incorporation of carbon isotopes was stoichiometrically related to the ratio of labeled glucose to unlabeled glucose in the culture medium. The isotope-labeled variants retained the physicochemical properties of the unlabeled ELP, specifically its temperature dependent aggregation behavior. As an example of the utility of this method, the in vitro stability of 14C-labeled ELP in PBS and mouse serum was conveniently quantified by SDS-PAGE and autoradiography. In addition, the in vivo stability of the 14C-labeled ELP in plasma was determined along with its plasma pharmacokinetics.

Characterization of C-terminal-truncated urinary metabolites of a recombinant hirudin in rats

Although it has been reported that hirudin was excreted in urine mainly as its nonmetabolized form in humans, dogs, and rabbits, no report has been published about the molecular nature of urinary metabolites in rats. We found that nonmetabolized hirudin could not be detected in rat urine after its i.v. administration and that urinary metabolites of recombinant hirudin CX-397 consisted of at least the following six C-terminal-truncated peptides: CX-397(1-49), CX-397(1-50), CX-397(1-51), CX-397(1-52), CX-397(1-54), and CX-397(1-55), in the ratio of roughly 11, 51, 3, 11, 19, and 5%, respectively. In conclusion, the urinary metabolism of recombinant hirudin in rats is different from that in humans, dogs, and rabbits, suggesting that the handling of hirudin in rat kidney is unique among them.

Pharmacokinetics, absorption, distribution and disposition of [125I]-oligotide following intravenous or oral administration in the rat

Oligotide (O) was labelled with 125I. The radiolabelled compound ([125I]-Oligotide ([125I]-O)) retained the biological activity of parent O. Following single intravenous administration the half lives of radioactivity associated with O and/or O related components in plasma were 9-10 min and 9-10 h for alpha and beta phases respectively. Following single oral administration the half life of radioactivity associated with O and/or O related components in plasma was 11.45-12.76 h for beta fase. Following multiple oral administration once daily for 7 days, the half life of radioactivity associated with O and/or O related components following the 7th dose was 10-12 h for beta phase. The areas under plasma total radioactivity versus time curves were dose-dependent. Following single intravenous administration the major proportion of the administered dose was excreted via urine, while following single oral administration excretion via urine and faeces accounted for similar proportions of the administered dose. Following both single and oral administration the levels of radioactive components derived from [125I]-O in organs examined were generally highest in highly perfused organs.

Analysis of radiolabeled CHO cell-derived rHuGM-CSF pharmacokinetics and biodistribution in rhesus monkeys following intravenous and subcutaneous injection

The purpose of these studies was to examine the biodistribution and pharmacokinetics of radiolabeled human CHO cell-derived rHuGM-CSF in normal Rhesus monkeys (Macaca mulatta) following intravenous (i.v.) and subcutaneous (s.c.) injection. A dual radioisotope tracer technique was utilized to monitor the behavior of rHuGM-CSF in vivo. Recombinant HuGM-CSF was radiolabeled with I-123 (a 13.2 h half-life, 140 KeV pure gamma emitting radionuclide detected using gamma scintigraphic imaging) using a mild chloramine T reaction. A separate preparation of rHuGM-CSF radiolabeled with S-35 methionine by bioincorporation in tissue culture was mixed with the I-123-labeled protein, permitting comparison of data obtained from the two radiolabels. Two dose levels of rHuGM-CSF were used for i.v. bolus (15 and 300 micrograms/kg) and s.c. (10 and 100 micrograms/kg) studies. The results of these studies demonstrated that the co-administered I-123 rHuGM-CSF and S-35 rHuGM-CSF followed similar blood elimination kinetics after i.v. or s.c. injection. Following i.v. bolus injection, rHuGM-CSF was found to rapidly distribute to all central body cavity high blood flow organs, followed by rapid uptake in the kidneys and elimination in the urine. There were no differences in the pharmacokinetic values obtained for I-123- and S-35-labeled rHuGM-CSF nor for the two dose levels examined. Following, s.c. injection, I-123- and S-35-labeled rHuGM-CSF were found to reach maximal plasma levels after approximately 16 h. The primary route of elimination was the urine. Monkeys previously exposed to rHuGM-CSF were found to have circulating antibodies to rHuGM-CSF. Studies in these animals revealed a significantly altered distribution and clearance of radiolabeled rHuGM-CSF, with the majority of the injected activity being cleared by the liver.

Peptide stability in drug development. II. Effect of single amino acid substitution and glycosylation on peptide reactivity in human serum

The determination of peptide stability in human serum (HS) or plasma constitutes a powerful screening assay for eliminating unstable peptides from further development. Herein we report on the stability in HS of several major histocompatibility complex (MHC)-binding peptides. Some of these peptides are in development for the novel treatment of selected autoimmune disorders such as rheumatoid arthritis and insulin-dependent diabetes. For most of the l-amino acid peptides studied, the predominant degradation mechanism is exopeptidase-catalyzed cleavage. Peptides that were protected by d-amino acids at both termini were found to be more stable than predicted, based on additivity of single substitutions. In addition, N-acetylglucosamine glycopeptides were significantly stabilized, even when the glycosylation site was several amino acids from the predominant site(s) of cleavage. This indicates that long-range stabilization is possible, and likely due to altered peptide conformation. Finally, the effect of single amino acid substitutions on peptide stability in HS was determined using a model set of poly-Ala peptides which were protected from exopeptidase cleavage, allowing the study of endopeptidase cleavage pathways.

Turnover of synthetic class A amphipathic peptide analogues of exchangeable apolipoproteins in rats. Correlation with physical properties

Peptide analogues of the class A amphipathic helixes from exchangeable apolipoproteins mimic apolipoprotein (apo) A-I in a number of ways, including the ability to activate the enzyme lecithin:cholesterol acyltransferase, to associate with high density lipoproteins (HDLs), and to form HDL-like particles in the presence of lipids. This study investigated the metabolic properties of several of these peptide analogues in the rat. Peptide analogues studied were 18A (referred to as L-18A to differentiate it from D-18A, and which mimics apolipoprotein amphipathic helical domains in its charge distribution), 37pA (a dimer of two 18A monomers separated by a proline), 18R (with reversed charge distribution compared with 18A), and D-18A (identical in amino acid sequence to 18A but synthesized from D-amino acids). Peptides were radiolabeled with 125I. In addition, metabolism of rat and human 125I-apo A-I and human 14C-apo A-I was studied; no significant differences in clearance of these preparations were seen. Clearance data were fitted to multiexponential equations to give half-times of clearance; biexponential equations consistently provided the best nonlinear least-squares curve fit. The order of relative lipid affinity determined in vitro was 37pA greater than apo A-I greater than D-18A = L-18A greater than 18R. Half-times of clearance were in the same approximate rank order: 37pA, 6.9 +/- 3.3 hours (mean +/- SD); apo A-I, 6.9 +/- 1.8 hours; D-18A, 4.0 +/- 1.0 hours; L-18A, 4.6 +/- 1.6 hours; and 18R, 0.9 +/- 0.1 hour.(ABSTRACT TRUNCATED AT 250 WORDS)