Combined treatment with pemetrexed and vinflunine in patients with metastatic urothelial cell carcinoma after prior platinum-containing chemotherapy - results of an exploratory phase I study.

Vinflunine is to date the only registered agent for second-line treatment of metastatic urothelial cell carcinoma (UCC) in Europe. However, the effect is modest. Pemetrexed has demonstrated some single-agent activity in this disease entity. In order to improve treatment possibilities for UCC patients, a phase I trial (VINTREX) was undertaken to assess the safety of vinflunine and pemetrexed in metastatic UCC patients. A dose escalation design was planned to determine the dose-limiting toxicity (DLT) and maximum tolerated dose (MTD) of a vinflunine/pemetrexed combination. Pemetrexed was added to vinflunine dosed at 280 mg/m2 on day 1 of a 21-day cycle. Three levels of pemetrexed were planned starting at 400 mg/m2. Four patients were enrolled with a mean age of 66 years and with a mean number of prior GC-cycles of 6,8. Two DLT's were observed at the lowest dose-level in cohort 1. One patient experienced grade 4 thrombocytopenia and a second demonstrated hepatobiliary toxicity grade 3 with an increase in alanine aminotransaminase. Most common grade 3 and 4 adverse events were anemia, thrombocytopenia and neutropenia. Three out of four patients received 3 cycles of pemetrexed and vinflunine, all had progressive disease. Based on these observations and due to protocol design, the study was interrupted at dose level 1 for safety reasons. The combined therapy of vinflunine (Javlor®, Pierre Fabre Pharma) and pemetrexed (Alimta®, Eli Lilly) is poorly tolerated in metastatic UCC patients. The combination cannot be recommended for further investigations in metastatic UCC.

Targeting of distinct signaling cascades and cancer-associated fibroblasts define the efficacy of Sorafenib against prostate cancer cells.

Sorafenib, a multi-tyrosine kinase inhibitor, kills more effectively the non-metastatic prostate cancer cell line 22Rv1 than the highly metastatic prostate cancer cell line PC3. In 22Rv1 cells, constitutively active STAT3 and ERK are targeted by sorafenib, contrasting with PC3 cells, in which these kinases are not active. Notably, overexpression of a constitutively active MEK construct in 22Rv1 cells stimulates the sustained phosphorylation of Bad and protects from sorafenib-induced cell death. In PC3 cells, Src and AKT are constitutively activated and targeted by sorafenib, leading to an increase in Bim protein levels. Overexpression of constitutively active AKT or knockdown of Bim protects PC3 cells from sorafenib-induced killing. In both PC3 and 22Rv1 cells, Mcl-1 depletion is required for the induction of cell death by sorafenib as transient overexpression of Mcl-1 is protective. Interestingly, co-culturing of primary cancer-associated fibroblasts (CAFs) with 22Rv1 or PC3 cells protected the cancer cells from sorafenib-induced cell death, and this protection was largely overcome by co-administration of the Bcl-2 antagonist, ABT737. In summary, the differential tyrosine kinase profile of prostate cancer cells defines the cytotoxic efficacy of sorafenib and this profile is modulated by CAFs to promote resistance. The combination of sorafenib with Bcl-2 antagonists, such as ABT737, may constitute a promising therapeutic strategy against prostate cancer.

Current status of prognostic immunohistochemical markers for urothelial bladder cancer.

The management and prognostication of patients with urothelial carcinomas (UCs), the most common histological type of bladder cancer, is mainly based on clinicopathological parameters. Several markers have been proposed to monitor this disease, including individual cell cycle-related proteins such as p53, pRb, p16, p21 and p27. Other putative markers are the oncogene products of FGFR3 and the ErbB family, proliferation markers including Ki-67, Aurora-A and survivin and different components within the immune system. In this review, a total of 12 parameters were evaluated and their discriminatory power compared. It is concluded that, in single-marker analyses, the proliferation markers Ki-67, survivin and Aurora-A offer the best potential to predict disease progression since they were all able to demonstrate independent prognostic power in repeated studies. Markers related to the immune system (e.g. CD8+ cells, regulatory T cells and cyclooxygenase-2 expression) or oncogene products of the ErbB family and FGFR3 are less powerful predictors of outcome or have not been equally well studied. The cell cycle-related proteins p53, pRb, p16, p21 and p27 have been extensively studied, but their usefulness as single prognostic markers remains unclear. However, in multimarker analyses, these markers appear to add prognostic information, indicating that they may contribute to more accurate treatment of UC.

High dose-rate brachytherapy of prostate cancer utilising Iridium-192 after-loading technique: technical and methodological aspects.

The aim of this study was to focus on certain characteristic problems associated with Iridium-192 high dose-rate brachytherapy (Ir-192 HDR-BT) in combination with external beam radiation therapy (EBRT) in the treatment of patients with localised prostate cancer. Over a period of 16 years, >2,000 patients with prostate cancer have been treated in Sweden with a combination of two fractions of 10 Gy Ir-192 HDR-BT and 50 Gy of fractionated EBRT. Although this treatment is usually well tolerated, there are biological and technical factors to be considered before and during the treatment of the patient to avoid side effects or under-treatment of the target volume. Some of the problems facing the doctors are transducer stability, needle deviation, target definition, target motion, pubic arch interference, concomitant diseases and tolerance doses for different organs at risk. These problems are discussed and possible solutions are presented in this study.

A new specific method for measuring S-100B in serum.

The S-100 family of proteins are acidic calcium and zinc binding low molecular weight proteins mainly present in astrocytes and in a population of oligodendrocytes of the CNS. S100b, an acidic low weight and zinc binding protein, has attracted considerable interest due to its release into the cerebrospinal fluid and blood from brain tissue following brain damage and from malignant melanomas. A new simple two-step incubation assay has now been elaborated in which two catcher and one tracer monoclonal antibodies are used. The specificity of this assay is high because all the MAbs used bind exclusively to S-100B, as shown by real-time biospecific interaction analyses. Moreover, the working range of the assay is 0.2-60 micrograms/L with a CV of less than 10%; the resulting high sensitivity has been confirmed by clinical studies. Time dependence, shaking conditions, lower limit of detection limits, effects of dilution, hook effect, recovery, impression as intra- and interassay variations, and crossreactivities with S-100A1 were tested in order to obtain a highly reproducible assay. Sera from healthy blood donors and patients undergoing cardiopulmonary bypass operations were tested with the assay. Several of the patients undergoing open heart surgery presented measurable values in this IRMA S-100 assay, indicating cerebral effects of open heart surgery. The test may be used for postoperative monitoring of these patients.

Idiotypic-anti-idiotypic antibody interactions in experimental radioimmunotargeting.

Idiotypic-anti-idiotypic antibody interactions can be used in vivo to regulate the serum levels of specific radiolabeled antibodies. Anti-idiotypic antibodies can also be used as clearing agents for radiolabeled antibodies in radioimmunolocalization and radioimmunotherapy. The present study describes the immunochemical interactions between the monoclonal idiotype (H7) and three generated monoclonal anti-idiotypic antibodies (alphaH7:1, alphaH7:35, and alphaH7:38). An unexpected variability in complex formation could be demonstrated in vitro, revealing three different stable complex patterns, i.e., low molecular weight 1:1 complexes, ladder formation with oligomeric, consecutively added constituents, and large linear polymeric complexes of high molecular weight. Within 24 h, the anti-idiotypes were able to cause a significant decrease in total body radioactivity, and the antibody generating a ladder formation (alphaH7:38) was found to be the most efficient at removing radiolabeled idiotypes from the circulation. It is concluded that monoclonal anti-idiotypic antibodies may be valuable tools in improving radioimmunolocalization and radioimmunotargeting.

H-2Dd engagement of Ly49A leads directly to Ly49A phosphorylation and recruitment of SHP1.

We have used a number of in vitro and in vivo techniques to identify the molecules that can bind to the cytoplasmic tail of the Ly49A receptor. Affinity chromatography using peptides corresponding to the N-terminal 18 amino acids of Ly49A allowed the recovery of a number of proteins that bound preferentially to the tyrosine-phosphorylated peptide, including SH2-containing phosphatase-1 (SHP1) and the SH2-containing inositol 5' phosphatase (SHIP). In another approach, using the entire cytoplasmic domain of the Ly49A receptor, we found that SHP2 also interacted with the tyrosine-phosphorylated form of the Ly49A cytoplasmic tail. Using BIACORE(R)2000 analysis, we determined that both SHP1 and SHP2 bound to the tyrosine-phosphorylated cytoplasmic tail of Ly49A with affinities in the nanomolar range, whilst SHIP showed no binding. Mutation of tyrosine-36 to phenylalanine did not significantly affect the affinities of these proteins for the tyrosine-phosphorylated cytoplasmic tail of Ly49A. In addition, using a whole-cell system with T-cell lymphoma cell lines that expressed the Ly49A receptor or its H-2Dd ligand, we determined that engagement of Ly49A by its major histocompatibility complex (MHC) ligand leads to tyrosine-phosphorylation events and recruitment of SHP1. Recruitment of SHP1 was rapid and transient, reaching a maximum after 5 min. These data suggest that mechanisms for the inhibitory signal are generated following receptor engagement. They also provide direct evidence that ligand engagement of the Ly49A receptor is responsible for recruitment of downstream signalling molecules.

Stability and immunoreactivity of the monoclonal anticytokeratin antibody TS1 after different degrees of iodination.

The immunoreactivity, stability and in vivo kinetics of an anticytokeratin 8 monoclonal antibody, TS1, were investigated following different degrees of labeling with 125I (0.2, 1 and 2-3 125I/TS1 MAb). By testing with ELISA, it was demonstrated that a high degree of iodination, i.e. > 2 125I/TS1, caused a rapid decrease in immunoreactivity to almost zero within 10 days. Furthermore, a complete degradation to low molecular weight fragments and free iodine was seen, as shown by SDS PAGE and autoradiography. The differently labeled radionuclide conjugates were injected into nude mice inoculated with HeLa Hep2 cells and tumor doses (estimated by MIRD formalism), tumor:non-tumor dose ratios, % I.D./gram tissue, Gy/MBq and in vivo kinetics of the differently labeled MAbs were determined. Despite the in vitro instability of the highest iodinated radionuclide conjugate, it was possible to deliver high doses to the tumors if the conjugate was injected into the animal immediately after completion of the iodination procedure. Increases from 1.4 Gy to 15.2 Gy delivered tumor dose were obtained with a tenfold increase in the specific activity, without alterations in the tumor:non-tumor tissue dose ratios. There is room for significant improvements in efficacy at radioimmunotherapy, which can be gained by optimizing the degree of iodination. For therapeutical applications a high degree of iodination may be an advantage.

Epitope specificity of the monoclonal anticytokeratin antibody TS1.

Due to their abundance in epithelial cells and deposition in necrotic regions intratumorally, cytokeratins (CKs) have been established as valuable targets for both radioimmunolocalization and radioimmunotherapy. The target epitope for the monoclonal anti-CK8 antibody, TS1, used for both experimental radioimmunolocalization and radioimmunotherapy, was determined by means of synthesis of 96 overlapping peptides that covered the entire CK8 molecule. A highly conserved peptide sequence, spanning amino acids (aa) 343-357 and covering the discontinuous epitope in the helical 2B domain, was identified. The epitope retains its helical structure, as shown with circular dichroism spectroscopy, although the length of the peptide (ie., >20 aa) is crucial for maintenance of immunoreactivity. To determine which aa residues are crucial for binding to the monoclonal antibody, alanine scanning was performed on a 26-mer covering aa 340-365, with the sequence QRGELAIKDANAKLSELEAALQRAKQ. The 26 modified peptides were evaluated using ELISA and BIAcore technology. The uniqueness of this epitope has been established by data base sequence comparisons.

Endogenous anti-tumor antibody responses in nude mice.

BACKGROUND: Nude mice with xenografted human tumors is the most exploited animal model used to elucidate the efficacy of experimental radioimmunolocalization and radioimmunotherapy. These animals accept transplants and are generally considered immunologically inert with regard to cell-mediated and humoral immune responses against the tumors. METHODS: Nude control mice and mice carrying human HeLa Hep-2 tumor xenografts were studied for appearance of endogenous antibodies following inoculation with tumor cells. The titers of these antibodies were investigated by isotype-specific enzyme-linked immunosorbent assay (ELISA) technologies, fluorescence-activated cell sorter analysis (FACS), BIAcore (Pharmacia Biosensor AB, Uppsala, Sweden) technology, and immunofluorescence. RESULTS: The HeLa Hep-2 cell line was found to be immunogenic in all investigated animals by means of ELISA, FACS, and BIAcore evaluations as well as by immunofluorescence against both tested antigens, placental alkaline phosphatase and cytokeratin 8. Predominantly immunoglobulin M antibodies were induced, but immunoglobulin G isotypes could also be identified. Sera from these tumor-bearing mice were used for immunohistochemistry of the tumor cells. The antibodies seemed to be of low affinity and may be displaced by high-affinity monoclonal antibodies used in radioimmunotargeting. CONCLUSIONS: Nude mice bearing tumor xenografts produce significant amounts of antibodies against these two human tumor-derived antigens. These endogenous antibodies may influence targeting of radiolabeled antibodies. They also have the potential to interfere with the pharmacokinetics of labeled or nonlabeled idiotypic antibodies during experimental immunolocalization.

Dosimetry of fractionated administration of 125I-labeled antibody at experimental radioimmunotargeting.

BACKGROUND: Radiotherapy of solid tumors is preferably performed in fractionated doses. Conversely, radioimmunotherapy with nuclide-carrying antibodies delivers a continuously decreasing low dose rate during a longer time period after a single injection. In the current study, the same total amount of 125I-labeled anticytokeratin monoclonal antibody (MoAb) was administrated in one, three, or ten injections and the dosimetry was evaluated. METHODS: Three groups of nude mice (10 mice each) with HeLa Hep 2 xenografts were injected with 1 x 100 microg/22.2 megabecquerel (MBq), 3 x 33 microg/7.4 MBq, and 10 x 10 microg/2.22 MBq 125I-labeled TS1 MoAb, respectively. The mice were examined scintigraphically over a 54-day period (total number of radio immunoscintigraphies (RISs) = approximately 700) and doses to tumor and normal tissues were estimated according to the medical internal radiation dose formalism. RESULTS: A single bolus injection caused higher tumor uptake, tumor dose, and tumor to nontumor dose ratio than administration of the same total dose of antibody and radioactivity in three or ten separate injections. The single bolus injection caused a tenfold higher tumor uptake (% injected dose, or ID) compared with the group receiving ten injections. This caused a tumor dose of 17 gray to the group receiving a single bolus injection. CONCLUSIONS: In this antigen target system, a single injection of a large amount of antibody was found to be more efficient than the same antibody dose subdivided into three or ten fractions. It was concluded that not only the radioactivity but also the amount of antibody per fraction should be considered when determining optimal fractionated radioimmunotherapy.

Dosimetry of fractionated experimental radioimmunotargeting with idiotypic and anti-idiotypic anticytokeratin antibodies.

BACKGROUND: Repeated injections of iodine-125 (125I)-labeled tumor targeting anticytokeratin monoclonal antibody (TS1) and a nonlabeled antiidiotypic monoclonal antibody against TS1 (alphaTS1) were compared with a single injection of the radiolabeled TS1 in experimental radioimmunotargeting. Anti-TS1 was used to remove nontargeting TS1. METHODS: Nude mice were inoculated with HeLa Hep2 cells. The animals in Group A received a single injection of 13 MBq 125I-labeled TS1. The animals in Group B received four injections of 125I-labeled TS1 (8-13 MBq) followed by alphaTS1 24 hours later, at 2-week intervals. The mean absorbed doses were calculated according to the Medical Internal Radiation Dose Committee criteria based on repetitive radioimmunoscintigraphies during an observation period of 59 days. RESULTS: A 11 gray (Gy) mean dose to the tumor and 2 Gy to the whole body was achieved in Group A. Mean peak tumor uptake of 5% of the injected dose (ID), corresponding to 14% ID/g, was observed on Day 17 after a single injection of the labeled monoclonal antibody. A mean peak tumor uptake of the same order of magnitude was seen in Group B. An absolute increase in the tumor uptake was observed in Group B during the entire observation period. The mean absorbed dose to the tumors was 11 Gy at the end of the observation period, whereas the whole body dose was only 2.5 Gy in Group B. Autoradiography of the tumors at the end of the observation period confirmed an intensive heterogeneous accumulation of activity in the entire tumor. CONCLUSIONS: The fractionated strategy can contribute to a significant accumulation of radiolabeled TS1 in the tumors. Furthermore, the use of alphaTS1 makes it possible to increase the tumor-to-nontumor dose ratio and maintain a prolonged high activity accumulation in the tumor.

Experimental radioimmunotargeting combining nonlabeled, iodine-125-labeled, and anti-idiotypic anticytokeratin monoclonal antibodies: a dosimetric evaluation.

BACKGROUND: Preinjection of a nonlabeled tumor targeting anticytokeratin monoclonal antibody (TS1) and postinjection of an anti-idiotypic anticytokeratin monoclonal antibody (alphaTS1) were evaluated separately and in combination to investigate their effects on the accumulation of iodine-125 (125I)-labeled TS1 in experimental radioimmunotargeting. TS1 targets deposited extracellular cytokeratin 8 from necrotic tumor cells. METHODS: Nude mice were inoculated with HeLa Hep 2 cells. Four different groups were followed with 504 repetitive quantitative radioimmunoscintigraphic recordings during a 78-day observation period. The absorbed doses were calculated according to criteria of the Medical International Radiation Dose Committee. RESULTS: As much as 2% of the injected dose (ID) of 125I-labeled TS1 accumulated in the tumor, and the peak tumor uptake was recorded as late as Day 30 after the injection of 125I-labeled TS1. Anti-TS1 caused a rapid decrease in the whole body activity. The highest tumor-to-nontumor activity ratios were obtained when a pre-injection of nonlabeled TS1 was combined with a postinjection of alphaTS1. The mean absorbed dose in tumor per unit activity administered was 0.44 gray/megabecquerel (Gy/MBq) and in nontumor tissues 0.15 Gy/MBq after a single injection of 125I-TS1. The efficacy was 0.34 Gy/MBq in tumor and 0.1 Gy/MBq in nontumor tissues after a combination of preinjection of nonlabeled TS1 and postinjection of nonlabeled alphaTS1. This indicates a 20% increase in tumor doses compared with a single injection of labeled TS1. CONCLUSIONS: This study confirms an extensive accumulation of TS1 in the tumor, with peak values as late as 30 days after injection of labeled TS1. Furthermore, both preinjection of nonlabeled TS1 and postinjection of alphaTS1 can improve radioimmunotargeting.

Analyte and label binding assay read by flow cytometry.

A new immunometric two-site sandwich assay is introduced, in which a label-scavenging binding partner is added to the sample in addition to the analyte-binding partner. The scavenger binding partner binds excess label antibody, giving a signal proportional to the amount of excess label antibody in the sample solution. A set of two calibration curves is obtained from the two binding partners simultaneously, and a combination of the two signals gives an unambiguous determination of the analyte concentration, even for high analyte concentrations where the hook effect may occur. Two-particle immunofluorometric assays developed for placental alkaline phosphatase and human chorionic gonadotropin on the basis of this principle and yielding signals measured by flow cytometry gave rapid results (2 h) and had working ranges in excess of 5 and 6 orders of magnitude for the respective analytes.

Twenty years with monoclonal antibodies: State of the art--Where do we go?

In this review, we have selected some parameters with the potential to improve the efficacy of RIL and RIT. Focus has partially been on the behaviour of radiolabelled antibodies in vivo in relation to properties and amounts of both target antigen and the antibodies used. If, out of the 28 factors listed in Table 1, some should be given preference in future work, it is our opinion that after the initial saturation of the tumour site a rapid decrease in redundant antibody is of significant importance. Furthermore, quantitative aspects of both antigens and antibodies should be more carefully evaluated when possible. By combining several of the listed approaches toward increasing efficiency, a more extensive use of RIL and RIT could be expected in the future.

Secondary antibodies as tools to improve tumor to non tumor ratio at radioimmunolocalisation and radioimmunotherapy.

One way of selectively improving the efficiency of radioimmunolocalization and radioimmunotherapy is to eliminate redundant, circulating, non-targeting radiolabeled antibodies after saturation of the target sites. Secondary antibodies of different types have been proposed as clearing agents for such purposes. The conceptually different approaches of the 'secondary antibody' strategy including its advantages and limitations are discussed. This mini-review also presents a model describing the kinetics of the components (the antigen, the primary and secondary antibodies) and approaches required to improve the efficacy of both radioimmunolocalization and radioimmunotherapy.

A putative mechanism for the non-specific uptake of intact radiolabelled monoclonal antibodies in the testes and prostate.

Non-specific testicular accumulation of radiolabelled intact anti-CEA monoclonal antibody (MAb), (A431/26, Behringwerke AG) was observed in 11 out of 12 patients with the testes and prostate included in the examination field at radioimmunoscintigraphy (RIS). Previous studies have shown that placental alkaline phosphatase (PLAP) serves as an Fc-receptor, mediating IgG transport through the placenta. A closely related protein, the germ cell alkaline phosphatase (GCAP), is expressed in the testes. The testicular uptake of IgG is observed only when intact but not fragmented MAbs are used, indicating involvement of Fc-receptors. MDCK cells (dog kidney cell line) transfected with the plasmid pSVT7 containing the GCAP gene were shown to acquire the capacity to both express membrane bound GCAP and to bind IgG on the cell surface. This might indicate that GCAP is responsible for the non-specific accumulation of intact MAb in the testes and prostate often observed when intact murine MAbs are used for radioimmunolocalization (RIL).

Use of anticytokeratin monoclonal anti-idiotypic antibodies to improve tumor:nontumor ratio in experimental radioimmunolocalization.

A syngeneic, high-affinity, anti-idiotypic monoclonal antibody (MAb; alpha TS1) raised against an anticytokeratin monoclonal antibody (TS1) was evaluated as a second antibody to promote the rapid clearance of radiolabeled TS1 from the blood during experimental radioimmunolocalization. By using a novel biosensor technology (BIAcore), association rate dissociation rate, and affinity constants between the idiotype and the anti-idiotype could be determined. The in vivo results in nude mice carrying HeLa Hep 2 tumors demonstrate the possibility of selectively regulating the amount of the idiotypic 125I-labeled circulating MAb by in vivo injection of this high-affinity, anti-idiotypic antibody. Injection of the anti-idiotype in a molar ratio of 0.75 to the idiotype cleared the blood pool from circulating radiolabeled idiotype within 24 h, with a concomitant rapid excretion of 125I in urine. The total amount of remaining radioactivity in the animals decreased to 15-20% during these 24 h, with the tumors still retaining 60-65% of their initial radioactivity. This approach, using syngeneic primary and secondary MAbs with minimized immunogenicity, significantly improves the tumor:nontumor ratio, thus improving efficiency in experimental radioimmunolocalization and radioimmunotherapy, leaving the endogenous antibody repertoire of the host unaffected.

Syngeneic anti-idiotypic antibodies eliminate excess radiolabeled idiotypes at experimental radioimmunolocalization.

Significant improvements in tumor/nontumor ratio can be achieved by injections of nonlabeled anti-idiotypic monoclonal antibodies (MAbs) during radioimmunolocalization and radioimmunotherapy using MAbs to target experimental tumors. The in vivo effects of an anti-idiotypic MAb (alpha H7) against a radioiodinated, high affinity, low dissociation rate, monoclonal antiplacental alkaline phosphatase antibody (H7) was investigated. Following in vivo injection of the anti-idiotypic MAb, the radioactivity in experimental tumors was found to decrease only 25% while the reduction of corresponding radioactivity in nontumor tissues amounted to 65-85%, compared to the group receiving no anti-idiotypic MAbs. These results indicate that it is possible to partially clear the circulation and nontumor tissues from excess of radiolabeled idiotypic antibody, without significant decrease in specific tumor localization, increasing the tumor/nontumor ratio three- to fourfold. Circulating nontumor targeting radiolabeled antibodies is one of the major limiting factors in radioimmunotherapy today. Injection of anti-idiotypic MAbs could selectively significantly reduce the radiation dose to radio-sensitive tissues, i.e., bone marrow and intestine, thus improving efficiency in radioimmunoscintigraphy and radioimmunotherapy.

In vivo and in vitro interactions between idiotypic and antiidiotypic monoclonal antibodies against placental alkaline phosphatase.

A monoclonal antiidiotypic antibody alpha H7, was generated against a monoclonal antibody H7 with specificity towards placental alkaline phosphatase. The in vitro and in vivo effects of alpha H7 were investigated. The antiidiotypic antibody was found to generate stable complexes with the radiolabeled idiotypic antibody, visualized both in vivo and in vitro, as revealed by PAGE and autoradiography. Using biosensor technology (BIAcore, Pharmacia) the interactions were followed in real time and the association rate, dissociation rate, and affinity constants between the reactants were determined (KA H7/PLAP 6.7 x 10(9) M-1, KA H7/alpha H7 3.2 x 10(9) M-1). By in vivo injection of the antiidiotype, a rapid dose dependent clearance of circulating radiolabeled idiotypes was demonstrated and a decrease in total body radioactivity was recorded with a concomitant dramatic increase in non-protein-bound 125I excreted in the urine. It is concluded that idiotypic-antiidiotypic interactions offer advantages in the regulation of antibody levels in vivo.