Production of an anti-prostate-specific antigen single-chain antibody fragment from Pichia pastoris.

Prostate-specific antigen (PSA) is a widely used marker for screening and monitoring prostate cancer. Because PSA levels are normally quite low, an antibody-based assay must be used to detect PSA. However, not all PSA-specific antibodies bind equally well to PSA or to its different isoforms. Therefore, a better understanding of how PSA interacts with PSA-specific antibodies is of considerable clinical interest. B80.3 is a widely used murine monoclonal anti-PSA antibody (IgG), which has very high affinity for both free and alpha-anti-chymotrypsin complexed PSA. More importantly, its gene sequence is known-making it one of only two anti-PSA antibodies that has been fully cloned and sequenced. To better elucidate the interaction between PSA and B80.3, a single-chain antibody fragment, derived from the variable domain of B80.3 (scFvB80), was cloned into a pPIC9 vector and expressed in Pichia pastoris. The secreted protein was purified using a three-step protocol beginning with a 50% ammonium sulfate precipitation step, followed by a T-gel thio-affinity step and concluding with a simple anion-exchange (DE52) filtration step. NMR studies indicate the protein is correctly folded while competitive enzyme-linked immunosorbant assays show that the purified scFvB80 has approximately 20% of the activity of the full-length B80.3 antibody. The protocol described here provides a quick and convenient route to prepare large quantities of very pure anti-PSA antibody fragments (15-20 mg/L culture medium) for detailed structural and biophysical characterization.

Summary report of the TD-3 workshop: characterization of 83 antibodies against prostate-specific antigen.

Twelve research groups participated in the ISOBM TD-3 Workshop in which the reactivity and specificity of 83 antibodies against prostate-specific antigen (PSA) were investigated. Using a variety of techniques including cross-inhibition assays, Western blotting, BIAcore, immunoradiometric assays and immunohistochemistry, the antibodies were categorized into six major groups which formed the basis for mapping onto two- and three-dimensional (2-D and 3-D) models of PSA. The overall findings of the TD-3 Workshop are summarized in this report. In agreement with all participating groups, three main antigenic domains were identified: free PSA-specific epitopes located in or close to amino acids 86-91; discontinuous epitopes specific for PSA without human kallikrein (hK2) cross-reactivity located at or close to amino acids 158-163; and continuous or linear epitopes shared between PSA and hK2 located close to amino acids 3-11. In addition, several minor and partly overlapping domains were also identified. Clearly, the characterization of antibodies from this workshop and the location of their epitopes on the 3-D model of PSA illustrate the importance of selecting appropriate antibody pairs for use in immunoassays. It is hoped that these findings and the epitope nomenclature described in this TD-3 Workshop are used as a standard for future evaluation of anti-PSA antibodies.

Relative affinity and specificity of monoclonal antibodies against prostate-specific antigen.

The relative affinities of a panel of antibodies submitted to the ISOBM TD-3 PSA Workshop were determined by direct-binding ELISA. The Workshop antibodies were also tested for reactivity with the prostate-specific antigen alpha1-antichymotrypsin complex (PSA-ACT) and cross-reactivity with porcine pancreatic kallikrein. There was a wide range of affinities observed for the panel of antibodies. Twelve antibodies failed to react with the PSA-ACT complex, and 1 antibody was found to recognize an epitope on ACT but not on PSA. Only 2 antibodies were found to react with porcine pancreatic kallikrein, a protein with 64% sequence homology with human glandular kallikrein-2 (hK2).

Epitope mapping of prostate-specific antigen with monoclonal antibodies.

Prostate-specific antigen (PSA) is a widely used marker for screening and monitoring prostate cancer. We identified and characterized the epitopes of two anti-PSA monoclonal antibodies (mAbs) designated B80 and B87. The epitopes were initially mapped as nonoverlapping by developing a sandwich immunoassay to measure PSA with the two anti-PSA mAbs. The two antibodies do not cross-react with homologous pancreatic kallikrein, but recognize epitopes unique to PSA. B80 and B87 can recognize both free and complexed PSA and hence measure total PSA. Epitope scanning and bacteriophage peptide library affinity selection procedures were used to identify and locate an epitope on PSA. A possible epitope for B80 was identified as being located on or near PSA amino acid residues 50-58 (-GRH-SLFHP-). The epitope for B87 was likely on an exposed nonlinear conformational determinant, unique to PSA, and not masked by the binding of B80 or alpha 1-antichymotrypsin.

Iodoazomycin riboside (1-(5'-iodo-5'-deoxyribofuranosyl)-2-nitroimidazole), a hypoxic cell marker. I. Synthesis and in vitro characterization.

Misonidazole (MISO), a selective radiosensitizer of hypoxic cells, forms adducts with cellular biomolecules with rates which are 30-50 X higher under hypoxic as compared to aerobic conditions of incubation. This technique of sensitizer adduct formation was proposed as a possible means of measuring the hypoxic fraction of solid tumors by noninvasive procedures. Iodoazomycin riboside (5'-IAZR) and 5'-[125I]AZR were synthesized and chemically characterized. Measurements of in vitro cytotoxicity and radiosensitizing ability with EMT-6 tumor cells in vitro indicated that 5'-IAZR is approximately 3 X more toxic and effective than is azomycin riboside (AZR) and approximately 10 X more toxic and effective than is MISO. 5'-[125I]AZR was shown to selectively bind to hypoxic EMT-6 cells at rates which were 2.5-3 X faster than those of MISO. The absolute rates of binding of 5'-IAZR to hypoxic cells at concentrations of 10-100 microM are the highest observed in this laboratory for any hypoxic cell radiosensitizer tested to date. These data suggest that 5'-IAZR, when labeled with an appropriate radioisotope (e.g., 131I), might be a useful marker for hypoxic cells in solid tumors amenable to noninvasive detection. Additional studies with animal tumor models appear to be warranted.

Electron-affinic compounds for labelling hypoxic cells: the synthesis and characterization of 1-[2-(2-iodophenoxy)-ethyl]-2-nitroimidazole.

Highly lipophilic and protein-bound derivatives of 1-(2-phenoxy-ethyl)-2-nitroimidazole (PENI; RGW-609), a known radiosensitizer, have been prepared by introducing one (IPENI) and two (DIPENI) iodine atoms into the phenyl ring via electrophilic substitution. The electron affinity of all 3 compounds, as determined by differential pulse polarography, was similar to that for MISO, a radiosensitizer which has undergone clinical trial, but P values were 2-3 orders of magnitude greater that for MISO and %PB values were as high as 30% in vitro compared to less than 1% for MISO. 131I-PENI was prepared by catalysed halogen exchange with Na131I in greater than 90% yield, and was found to be chemically and radiochemically stable in solution for at least 2 weeks. Whole-body studies in BDF/1 mice bearing EMT-6 tumors showed rapid hepatic extraction and biliary elimination with little or no accumulation in any other tissue including tumor and fat, indicating that P and % PB values had little impact on in vivo distribution and disposition. 131I-PENI was not measurably deiodinated in vivo. IPENI and DIPENI are radiosensitizers by inference only, that is, they have not been tested for radiosensitizer properties. However, PENI, the parent compound, has been shown to be active as a radiosensitizer, and the peak potentials (polarographic) for both IPENI and DIPENI fall near those of MISO, PENI and most other 2-nitroimidazole sensitizers. The low levels of concentration in target tissues achieved by 131I-PENI mitigate against its use as a diagnostic agent.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and in vivo studies of the radiosensitizer 4-[82Br]bromomisonidazole.

[82Br]Misonidazole ( [82Br]MISO) was investigated as a potential agent for scintillation imaging and indexing of hypoxic areas in tumor masses. [82Br]MISO was prepared by irradiating samples of Br-MISO in a SLOWPOKE reactor for 2 h at a thermal neutron flux of 10(12) n cm-2 s-1. Radiochemical yields ranged from 11 to 16%. [82Br]MISO had a plasma clearance half-life of approximately 40 min in BALB/c mice bearing EMT-6 tumors. The major metabolic product was identified as [82Br]Desmisonidazole. [82Br]MISO showed relatively high tumor uptake and persistently high plasma levels.