Palbociclib combined with endocrine treatment in hormone receptor-positive, HER2-negative breast cancer patients with high relapse risk after neoadjuvant chemotherapy: subgroup analyses of premenopausal patients in PENELOPE-B.

BACKGROUND: The PENELOPE-B study demonstrated that the addition of 1-year post-neoadjuvant palbociclib to endocrine therapy (ET) in patients with high-risk early breast cancer (BC) did not improve invasive disease-free survival (iDFS) compared to placebo. Here, we report results for premenopausal women. PATIENTS AND METHODS: Patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative BC at high risk of relapse [defined as no pathological complete response after neoadjuvant chemotherapy and a clinical, pathological stage, estrogen receptor, grading (CPS-EG) score ≥3 or 2/ypN+] were randomized to receive 13 cycles of palbociclib or placebo + standard ET. Ovarian function (OF) was evaluated by centrally assessed estradiol, follicle-stimulating hormone and anti-Müllerian hormone serum levels. RESULTS: Overall, 616 of 1250 randomized patients were premenopausal; of these, 30.0% were <40 years of age, 47.4% had four or more metastatic lymph nodes, and 58.2% had a CPS-EG score ≥3. 66.1% of patients were treated with tamoxifen alone, and 32.9% received ovarian function suppression (OFS) in addition to either tamoxifen or aromatase inhibitor (AI). After a median follow-up of 42.8 months (97.2% completeness) no difference in iDFS between palbociclib and placebo was observed [hazard ratio = 0.95, 95% confidence interval (CI) 0.69-1.30, P = 0.737]. The estimated 3-year iDFS rate was marginally higher in the palbociclib arm (80.6% versus 78.3%). Three year iDFS was higher in patients receiving AI than tamoxifen plus OFS or tamoxifen alone (86.0% versus 78.6% versus 78.0%). Patients receiving tamoxifen plus OFS showed a favorable iDFS with palbociclib (83.0% versus 74.1%, hazard ratio = 0.52, 95% CI 0.27-1.02, P = 0.057). Hematologic adverse events were more frequent with palbociclib (76.1% versus 1.9% grade 3-4, P < 0.001). Palbociclib seems not to negatively impact the OF throughout the treatment period. CONCLUSIONS: In premenopausal women, who received tamoxifen plus OFS as ET, the addition of palbociclib to ET results in a favorable iDFS. The safety profile seems favorable and in contrast to chemotherapy palbociclib does not impact OF throughout the treatment period.

Axitinib versus placebo as an adjuvant treatment of renal cell carcinoma: results from the phase III, randomized ATLAS trial.

Background: The ATLAS trial compared axitinib versus placebo in patients with locoregional renal cell carcinoma (RCC) at risk of recurrence after nephrectomy. Patients and methods: In a phase III, randomized, double-blind trial, patients had >50% clear-cell RCC, had undergone nephrectomy, and had no evidence of macroscopic residual or metastatic disease [independent review committee (IRC) confirmed]. The intent-to-treat population included all randomized patients [≥pT2 and/or N+, any Fuhrman grade (FG), Eastern Cooperative Oncology Group status 0/1]. Patients (stratified by risk group/country) received (1 : 1) oral twice-daily axitinib 5 mg or placebo for ≤3 years, with a 1-year minimum unless recurrence, occurrence of second primary malignancy, significant toxicity, or consent withdrawal. The primary end point was disease-free survival (DFS) per IRC. A prespecified DFS analysis in the highest-risk subpopulation (pT3, FG ≥ 3 or pT4 and/or N+, any T, any FG) was conducted. Results: A total of 724 patients (363 versus 361, axitinib versus placebo) were randomized from 8 May 2012, to 1 July 2016. The trial was stopped due to futility at a preplanned interim analysis at 203 DFS events. There was no significant difference in DFS per IRC [hazard ratio (HR) = 0.870; 95% confidence interval (CI) : 0.660-1.147; P = 0.3211). In the highest-risk subpopulation, a 36% and 27% reduction in risk of a DFS event (HR; 95% CI) was observed per investigator (0.641; 0.468-0.879; P = 0.0051), and by IRC (0.735; 0.525-1.028; P = 0.0704), respectively. Overall survival data were not mature. Similar adverse events (AEs; 99% versus 92%) and serious AEs (19% versus 14%), but more grade 3/4 AEs (61% versus 30%) were reported for axitinib versus placebo. Conclusions: ATLAS did not meet its primary end point; however, improvement in DFS per investigator was seen in the highest-risk subpopulation. No new safety signals were reported. Trial registration number: NCT01599754.

Clinical Trial Simulations From a Model-Based Meta-Analysis of Studies in Patients With Advanced Hepatocellular Carcinoma Receiving Antiangiogenic Therapy.

A mixed effect model describing median overall survival (mOS) in patients with advanced hepatocellular carcinoma (aHCC) treated with antiangiogenic therapy (AAT) was developed from literature data. Data were extracted from 59 studies, representing 4,813 patients. The final model included estimates of mOS after AAT (8.5 months) or placebo (7.1 months) administration. The mOS increased 21% when the AAT was sorafenib (SOR) or 42% when locoregional therapy was coadministered. The mOS decreased when patients received prior systemic therapy (↓7%) or concomitant chemotherapy (↓4%) or the percentage of patients with hepatitis B increased (↓∼0.4%/%). Clinical trial simulations of a phase II comparative trial predicted an mOS ratio (placebo:AAT) of 0.687 or 0.831, with a 65% or 22% probability of demonstrating superiority, for SOR or other AATs, respectively. Additionally, the 95% confidence interval (CI) of the simulated median mOS ratio for non-SOR AATs was similar to the 95% CI of the hazard ratio (HR) observed in the trial.

Randomized phase II study of axitinib versus placebo plus best supportive care in second-line treatment of advanced hepatocellular carcinoma.

BACKGROUND: The efficacy and safety of axitinib, a potent and selective vascular endothelial growth factor receptors 1-3 inhibitor, combined with best supportive care (BSC) was evaluated in a global, randomized, placebo-controlled phase II trial in patients with locally advanced or metastatic hepatocellular carcinoma (HCC). PATIENTS AND METHODS: Patients with HCC and Child-Pugh Class A who progressed on or were intolerant to one prior antiangiogenic therapy were stratified by tumour invasion (presence/absence of extrahepatic spread and/or vascular invasion) and region (Asian/non-Asian) and randomized (2:1) to axitinib/BSC (starting dose 5 mg twice-daily) or placebo/BSC. The primary end point was overall survival (OS). RESULTS: The estimated hazard ratio for OS was 0.907 [95% confidence interval (CI) 0.646-1.274; one-sided stratified P = 0.287] for axitinib/BSC (n = 134) versus placebo/BSC (n = 68), with the median (95% CI) of 12.7 (10.2-14.9) versus 9.7 (5.9-11.8) months, respectively. Results of prespecified subgroup analyses in Asian versus non-Asian patients or presence versus absence of tumour invasion were consistent with the overall population. Improvements favouring axitinib/BSC (P < 0.01) were observed in secondary efficacy end point analyses [progression-free survival (PFS), time to tumour progression (TTP), and clinical benefit rate (CBR)], and were retained among Asian patients in the prespecified subgroup analyses. Overall response rate did not differ significantly between treatments and patient-reported outcomes favoured placebo/BSC. Most common all-causality adverse events with axitinib/BSC were diarrhoea (54%), hypertension (54%), and decreased appetite (47%). Baseline serum analyses identified potential new prognostic (interleukin-6, E-selectin, interleukin-8, angiopoietin-2, migration inhibitory factor, and c-MET) or predictive (E-selectin and stromal-derived factor-1) factors for survival. CONCLUSIONS: Axitinib/BSC did not improve OS over placebo/BSC in the overall population or in stratification subgroups. However, axitinib/BSC resulted in significantly longer PFS and TTP and higher CBR, with acceptable toxicity in patients with advanced HCC. TRIAL REGISTRATION: ClinicalTrials.gov, NCT01210495.

Phase I study of sunitinib plus capecitabine/cisplatin or capecitabine/oxaliplatin in advanced gastric cancer.

BACKGROUND: We evaluated the maximum tolerated dose (MTD) and safety of sunitinib plus capecitabine/cisplatin (XP) or capecitabine/oxaliplatin (XELOX) in Korean patients with advanced gastric cancer (GC). METHODS: Sunitinib (37.5 or 25 mg/day) was administered on a 2-week-on/1-week-off schedule with chemotherapy. Assessments included dose-limiting toxicity (DLT), safety, pharmacokinetics, and antitumor activity. RESULTS: Twenty-eight patients received sunitinib/XP; 48 received sunitinib/XELOX. The MTDs were: sunitinib 25 mg/day, cisplatin 80 mg/m(2), and capecitabine 1,000 mg/m(2); sunitinib 37.5 mg/day, oxaliplatin 110 mg/m(2), and capecitabine 800 mg/m(2); and sunitinib 25 mg/day, oxaliplatin 110 mg/m(2), and capecitabine 1,000 mg/m(2). DLTs at the MTDs comprised grade (G) 4 febrile neutropenia plus G3 diarrhea (n = 1; sunitinib/XP), dose delays due to hematologic toxicity (n = 2; both sunitinib/XP), G3 bleeding (menorrhagia; n = 1; sunitinib/XELOX), and G3 increased alanine aminotransferase levels (n = 1; sunitinib/XELOX). There was a high frequency of G3/4 hematologic adverse events observed with both treatment regimens, particularly with sunitinib/XP. Frequent non-hematologic, G3/4 adverse events were nausea, stomatitis, and hypophosphatemia with sunitinib/XP and hypophosphatemia and pulmonary embolism with sunitinib/XELOX. No drug-drug interactions were apparent. At the MTDs, median progression-free survival was 6.4 months and 5.5-8.0 months for sunitinib/XP and sunitinib/XELOX, respectively; and the objective response rate was 46.7% and 43.5-45.5% for sunitinib/XP and sunitinib/XELOX, respectively. CONCLUSIONS: At the MTD, sunitinib/XELOX had an acceptable safety profile in patients with advanced GC.

Predicting the maximum-tolerated dose of PNU-159548 (4-demethoxy-3'-deamino-3'-aziridinyl-4'-methylsulphonyl-daunorubicin) in humans using CFU-GM clonogenic assays and prospective validation.

A haematotoxicity model was proposed by Parchment in 1998 to predict the maximum-tolerated dose (MTD) in humans of myelosuppressive antitumour agents by combining data from in vitro clonogenic assays on haematopoietic progenitors and in vivo systemic exposure data in animals. A prospective validation of this model in humans was performed with PNU-159548, a novel agent showing selective dose-limiting myelosuppression in animals. PNU-159548 and its main metabolite, PNU-169884, were tested in vitro on murine, canine and human colony forming units-granulocyte macrophages (CFU-GM) and in vivo on mice and dogs. The IC(90x) ratios (IC(x)=concentration inhibiting x% of colony growth) for CFU-GM and drug plasma protein binding were used to adjust the target plasma concentrations versus time curve (AUC) and predict the human MTD. The predicted MTD was compared with values achieved in phase I studies. Canine CFU-GM were 6-fold more sensitive (P<0.01) and murine CFU-GM 1.7-fold less sensitive (P<0.05) to PNU-159548 treatment than the human progenitors. PNU-169884 behaved similarly to PNU-159548. The predicted MTDs in humans calculated from data in mice and dogs were 15 and 38 mg/m(2), respectively. Overall, 61 patients were treated in two phase I studies, at doses ranging from 1.0 to 16 mg/m(2). Thrombocytopenia was dose-limiting with a MTD of 14 and 16 mg/m(2) in heavily and minimally pretreated/non-pretreated patients, respectively. Adjusting animal MTD data by means of the CFU-GM ratio between species can predict the human MTD with a good quantitative accuracy. Inhibition of common haemopoietic progenitors by PNU-159548 induced neutropenia/thrombocytopenia in animals and thrombocytopenia in patients, probably due to the higher sensitivity to the compound observed in human colony forming units-megakaryocyte (CFU-MK).

Phase I and pharmacokinetic studies of PNU-159548, a novel alkycycline, administered intravenously to patients with advanced solid tumours.

PNU-159548 (4-demethoxy-3'-deamino-3'-aziridinyl-4'-methylsulphonyl-daunorubicin) is the lead compound of a novel class of cytotoxic agents (alkycyclines) with a unique mechanism of action combining DNA intercalation with alkylation of guanines in the DNA major groove. The objectives of two phase I studies were to assess the dose-limiting toxicities (DLTs), to determine the maximum tolerated dose (MTD) and to study the pharmacokinetics (PKs) of PNU-159548 and its active metabolite PNU-169884 when administered intravenously (i.v.) over 10 or 60 min to patients with advanced solid tumours. Patients were treated with escalating doses of PNU-159548, courses repeated every 21 days at doses ranging from 1.0 to 16 mg/m(2). For pharmacokinetic analysis, plasma sampling was performed during the first course and assayed using a validated high-performance liquid chromatographic assay with mass spectrometric detection. 69 patients received a total of 161 courses. The MTD was reached at 14 and 16 mg/m(2) in heavily (HP) and minimally pretreated/non-pretreated (MP) patients, respectively, with thrombocytopenia as the DLT. A hypersensitivity reaction was observed in 8 patients across all dose levels, characterised by fever with chills, erythema, facial oedema and dyspnoea. The PKs of PNU-159548 and PNU-169884 were linear over the dose range studied. A significant correlation was observed between the percentage decrease in platelet count and the AUC of PNU-159548. In these studies, the DLT of PNU-159548 was thrombocytopenia. The recommended dose for phase II studies of PNU-159548 is 12 and 14 mg/m(2) administered i.v. over 10 min, once every 21 days, in HP and MP patients, respectively.

A prolonged methoxymorpholino doxorubicin (PNU-152243 or MMRDX) infusion schedule in patients with solid tumours: a phase 1 and pharmacokinetic study.

The aim of this phase I study was to assess feasibility, pharmacokinetics and toxicity of methoxymorpholino doxorubicin (MMRDX or PNU-152243) administered as a 3 h intravenous infusion once every 4 weeks. Fourteen patients with intrinsically anthracycline-resistant tumours received 37 cycles of MMRDX. The first cohort of patients was treated with 1 mg m(-2) of MMRDX. The next cohorts received 1.25 mg m(-2) and 1.5 mg m(-2) respectively. Common toxicity criteria (CTC) grade III/IV nausea and vomiting were observed in 1/18 cycles at 1.25 mg m(-2) and in 2/11 cycles at 1.5 mg m(-2). Transient elevation in transaminases up to CTC grade III was observed in 2/16 cycles at 1.25 mg m(-2) and 4/11 cycles at 1.5 mg m(-2). No cardiotoxicity was observed. At 1.25 mg m(-2) CTC grade IV neutropenia occurred in 1/17 cycles. At 1.5 mg m(-2) CTC grade III neutropenia was seen in 2/7 and grade IV in 3/7 evaluable cycles. Thrombocytopenia grade III was observed in 2/9 and grade IV in 1/9 evaluable cycles. One patient treated at 1.5 mg m(-2) died with neutropenic fever. Therefore, dose-limiting toxicity was reached and 1.25 mg m(-2) was considered the maximum tolerated dose for MMRDX as 3 h infusion. No tumour responses were observed. Pharmacokinetic parameters showed a rapid clearance of MMRDX from the circulation by an extensive tissue distribution. Renal excretion of the drug and its metabolite was negligible. In conclusion, prolongation of MMRDX infusion to 3 h does not improve the toxicity profile as compared with bolus administration.

Level of anti-mouse-antibody response induced by bi-specific monoclonal antibody OC/TR in ovarian-carcinoma patients is associated with longer survival.

More than 60% of cancer patients injected with intact murine monoclonal antibody (MAb) develop a humoral response against the antigen even after a single dose. Analysis of a series of 35 ovarian-cancer patients entered in phase-I and -II clinical studies of T-cells retargeted with the bi-specific F(ab')2 OC/TR revealed: (i) a detectable human anti-mouse antibody (HAMA) response in 31/35 (88%) patients, with high HAMA levels (> or = 150 ng/ml) in 18/31 (58%) cases by the end of the treatment; (ii) no correlation between HAMA levels and the form of delivery of the mAb (OC/TR bound to T cells or bound plus soluble), time schedule or cumulative dose; (iii) an association between high HAMA levels and favorable clinical parameters and response to immunotherapy; and (iv) a significantly longer median survival probability in patients with high HAMA levels than in patients with lower HAMA levels, even when the sub-group of non-responder patients was considered. Evaluation of the anti-idiotypic response in HAMA-positive sera indicated that 11/17 sera showed high-titer (>6000) binding of OC/TR, as evaluated by a specific radioimmunoassay, and 15/18 and 16/16 sera specifically inhibited the binding of the MOv18 and anti-CD3 parental MAbs to ovarian-carcinoma cells and T lymphocytes respectively. Of 7 patients evaluated for duration of the HAMA response, 5 showed stable or even increased HAMA levels. The long-lasting HAMA response maintained an anti-idiotypic component, directed mainly against the alphaCD3 idiotype of bi-MAb OC/TR in 2 out of 3 cases tested.

Efficiency of T cell triggering by anti-CD3 monoclonal antibodies (mAb) with potential usefulness in bispecific mAb generation.

T cell triggering can be achieved by monoclonal antibodies (mAbs) specific for the CD3/TcR complex. In the presence of appropriate costimulation and/or progression factors, such triggering permits the generation of effector cells for immunotherapy protocols involving the redirection of T cell lysis against tumor cells by mAbs bispecific for anti-CD3/anti-tumor cells (bs-mAbs). Focusing our analysis on the clinically relevant bs-mAb OC/TR, we found that bs-mAbs generated with the same anti tumor specificity, but two other anti-CD3 mAbs, TR66 and OKT3, have the same and a significantly lower lytic potential, respectively, compared with that of OC/TR. To evaluate the relevance of the anti-CD3 component, we examined several anti-CD3 mAbs with respect to binding parameters and the ability to trigger T lymphocytes. Competitive binding assays suggested that all anti-CD3 mAbs recognized the same or overlapping epitopes, although mAbs BMA030 and OC/TR bound with lower avidity than did alpha CD3 (the bivalent anti-CD3 mAb produced by the hybrid hybridoma OC/TR). TR66 and OKT3, as determined by measurement of the affinity constants. In all lymphocyte populations examined, which included resting peripheral blood mononuclear cells (PBMC), activated PBMC and T cell clones, OKT3, BMA033 and OC/TR failed to mobilize Ca2+ without cross-linking, whereas alpha CD3, in both murine and murine-human chimeric versions, TR66 and BMA030, did not require cross-linking. The ability to induce CD3 modulation was associated in part with the induction of Ca2+ fluxes. Despite the differences in the behavior of these mAbs in triggering the events that precede proliferation, all of them ultimately led to expression of the IL-2 receptor and to proliferation in T cells in the presence of accessory cells. Our data suggest that anti-CD3 mAbs that bind more rapidly (strong Ca2+ mobilizers) and more tightly under physiological conditions are good candidates for retargeting T cells in the bs-mAb clinical application.

Anti-idiotypic response to antigrowth factor receptor monoclonal antibodies.

The immunogenicity of the idiotypic portions of two antigrowth factor receptor monoclonal antibodies (MAbs) was studied. Immunisation of allogeneic but not syngeneic mice with antihuman epidermal growth factor receptor (EGF-R) MAb MINT5 or anti-HER-2/neu MGR6 MAb elicited a detectable titre of circulating antibodies, particularly when the MAb was coupled with the keyhole limpet haemocyanin and administered together with Freund's adjuvant. The anti-Ab1 response to MAb MINT5 was slightly delayed as compared with the response obtained with MAb MGR6 and was mainly directed to the variable regions. In both cases, all anti-Ab1-positive sera specifically competed with the binding of homologous radiolabelled Ab1 to the relevant EGF-R+ or HER-2/neu+ target cells. Fusion of splenocytes from MINT5-immunised animals failed to produce MAb, whereas cell fusion was successful in generating a paratope-related MAb in the case of MGR6. The anti-MGR6 MAb-produced IdM6.4 inhibited the binding of MAb MGR6 on breast carcinoma cells, suggesting that it recognises an idiotope in or near the antigen combining site, and can be considered useful in the identification and purification of the Ab1 or its derivatives. We analysed whether a possible recognition of murine EGF-R by MAb MINT5 or a mimicry of EGF by the MAb idiotype prevented or delayed the development of an idiotypic cascade in mice. MINT5 inhibited human and murine EGF binding to the human EGF-R, whereas the anti-Ab1 response competed with MINT5 but not with murine EGF binding to A431 human epidermoid carcinoma cells. Moreover, MINT5 did not recognise the murine EGF-R. In a phase I clinical study, no detectable levels of human antimouse antibody response were observed in 5 of the 6 treated cancer patients. The ability of MAb MINT5 to block human EGF-R function, together with its low immunogenicity in patients, raise the possibility of its application in carcinoma immunotherapy.

In vitro and in vivo stability and anti-tumour efficacy of an anti-EGFR/anti-CD3 F(ab')2 bispecific monoclonal antibody.

The in vitro and in vivo stability and anti-tumour efficacy of the anti-EGFR/anti-CD3 bispecific monoclonal antibody (biMAb), M26.1, were analysed. The interaction of the intact biMAb with Fc receptor I (Fc gamma RI) present on human leucocytes was not observed when the antibody was used as an F(ab')2 fragment. A CD8+ T-cell clone coated with M26.1 F(ab')2 was as effective as the intact biMAb in inducing IGROV1 target cell lysis when tested in a 51Cr-release assay. Variable levels of reduction of F(ab')2 to monovalent F(ab') were observed upon incubation with human ovarian cancer ascitic fluid (OCAF) or with human glioblastoma cavity fluid (GCF), but not with mouse or human sera. Activated lymphocytes coated with F(ab')2 and incubated in vitro with GCF or OCAF for 24 and 48 h respectively maintained their targeting. Thus, the F(ab')2, when present as a soluble molecule, but not when bound to T cells, might lose some functional activity as a consequence of partial reduction to F(ab'). In normal mice, M26.1 F(ab')2 retained full cytotoxic activity in the circulation, and clearance values were similar to those obtained with parental and other MAb F(ab')2. Treatment of IGROV1 tumour-bearing mice with activated human lymphocytes coated with the M26.1 F(ab')2 significantly prolonged survival of the animals compared with tumour-bearing untreated and control mice treated with lymphocytes or F(ab')2 alone. Together, these results suggest the clinical usefulness of bispecific M26.1 F(ab')2 as a targeting agent for local treatment of tumours such as glioma and ovarian cancers that express variable levels of epidermal growth factor receptor (EGFR).

Anti-tumor efficacy of an anti-epidermal-growth-factor-receptor monoclonal antibody and its F(ab')2 fragment against high- and low-EGFR-expressing carcinomas in nude mice.

Monoclonal antibody (MAb) MINT5 specifically detects the epidermal-growth-factor receptor (EGFR). In vitro analyses of intact MINT5 (IgG1) and its F(ab')2 fragment indicated that both forms of the MAb inhibited binding of 125I-mEGF to EGFR, induced receptor internalization and blocked EGF-induced EGFR tyrosine-kinase activation in A431 cells. Both forms of the MAb also inhibited to the same extent the proliferation of the carcinoma cell lines A431 and IGROVI, despite the difference in EGFR levels on the cells. The detection of TGF alpha mRNA and the inhibition of cell growth in EGF-free conditions by anti-EGFR MAb indicated the involvement of an EGFR/TGF alpha autocrine/paracrine pathway in the in vitro growth of both cell lines. Analysis of mice xenotransplanted s.c. with A431 cells and treated with MINT5 revealed a block in A431 tumor take in 6 of 10 animals when intact MAb was administered from day 0 to day 11. On a molar basis, F(ab')2 at the same dose was ineffective, although at a 7-fold higher dose F(ab')2 reduced s.c. tumor growth by 80%. At the same dose, intact MINT5 MAb reduced s.c. growth of the EGFR-negative MeWo cell line by 5%. Survival of mice bearing IGROVI i.p. xenotransplants and treated locally with either form of MAb was significantly prolonged even when treatment was initiated on day 3. Corrected doses of intact and F(ab')2 fragment, which accounted for the difference in serum half-lives of the MAb forms, resulted in similar survival rates in the tumor-bearing mice. These pre-clinical results suggest that MINT5 MAb might be safely used for systemic therapy of EGFR-over-expressing tumors. Loco-regional therapy might be contemplated in the case of tumors with moderate/low EGFR expression.

Unidirectional potentiation of binding between two anti-FBP MAbs: evaluation of the involved mechanisms.

The monoclonal antibody MOv19 directed to a folate binding protein shows temperature-dependent potentiation of binding of the noncompeting monoclonal antibody MOv18 to the relevant antigen, but the mechanism involved in this phenomenon had remained unclear. Use of chimeric versions of both monoclonal antibodies and the F(ab')2 and Fab fragments of MOv19 revealed an increment in MOv18 binding in all combinations irrespective of the origin of the Fc portion of the monoclonal antibody. The potentiating effect of bivalent MOv19 fragments on 125I-MOv18 binding was similar to that of the entire monoclonal antibody and occurred at saturating concentrations of both reagents at which monovalent binding prevails. Similarly, the monovalent fragment also induced a significant increase in MOv18 binding. However, the potentiation occurred only at very high concentrations of antibody fragment. Homologous inhibition was drastically reduced using MOv19 Fab fragment, suggesting a low binding stability of the monovalent reagent. Immunoblotting analysis and binding in the presence of exogenous purified folate binding protein indicated a cross-linking between soluble and cell surface molecules mediated by the bivalent monoclonal antibodies. The extent of the increase in MOv18 binding at 0 degrees C with high amounts of exogenous folate binding protein was lower than that obtained at 37 degrees C in the absence of added molecule. Release of 125I-MOv18 from the cell surface was significantly higher in the absence of MOv19 than in its presence.(ABSTRACT TRUNCATED AT 250 WORDS)