A multispecific anti-CD40 DARPin® construct induces tumor-selective CD40 activation and tumor regression

The CD40 receptor is an attractive target for cancer immunotherapy. Although a modest pharmacodynamic effect is seen in patients following administration of CD40-targeting monoclonal antibodies (mAb), the doses that could be safely administered do not result in a meaningful clinical response, most likely due to the limited therapeutic window associated with systemic CD40 activation. To overcome this issue, we developed a multispecific DARPin® construct, α-FAPxCD40, which has conditional activity at the site of disease. α-FAPxCD40 activation of CD40 depends on binding to fibroblast activation protein (FAP), a cell surface protease overexpressed in the stroma of solid tumors. In vitro studies demonstrated that α-FAPxCD40 potently activates human antigen-presenting cells in the presence, but not in the absence, of FAP-positive cells. After intravenous injection, a murine surrogate construct (α-mFAPxCD40) accumulated in FAP-positive tumors, elicited rejection of 88% of these tumors and induced memory anti-tumor immunity. Importantly, in contrast to the mouse anti-CD40 tested in parallel, the in vivo anti-tumor activity of α-mFAPxCD40 was neither associated with elevated blood cytokines nor with hepatotoxicity, both of which contribute to the clinical dose-limiting toxicities of several CD40 mAb. This study demonstrates that α-(m)FAPxCD40 engages CD40 in an FAP-restricted manner leading to tumor eradication without signs of peripheral toxicity. This distinct preclinical profile indicates that a favorable therapeutic index may be achieved in humans. It further supports the development of α-FAPxCD40, currently tested in a first-in-human clinical study in patients with solid tumors (NCT05098405).

Abstract 1073: A tumor-targeted CD40 agonistic DARPin® molecule leading to antitumor activity with limited systemic toxicity

CD40 is a member of the tumor necrosis factor receptor (TNFR) superfamily which can activate both innate and adaptive tumor immunity, making it an attractive target for cancer immunotherapy. Systemic administration of agonistic anti-CD40 antibodies has shown signs of activity in cancer patients, but dose-limiting toxicity has prevented the exploration of the full dose range and possibly has impaired clinical efficacy. To overcome this issue, we developed a multispecific DARPin® therapeutic candidate, MP0317, intended to activate CD40 receptor locally, rather than systemically, in the tumor microenvironment through cross-linking with fibroblast activation protein (FAP) that is abundantly present in the stroma of many solid tumors, but has limited expression in healthy adult tissues.

Using primary human 1) B cells and 2) dendritic cells, MP0317 activated the CD40 pathway in vitro. Upregulation of CD80, CD86, CD69 and IL12 occurred exclusively in the presence of FAP-positive, but not with FAP-negative cells, confirming a mechanism of action strictly dependent on FAP-mediated cross-linking.

A surrogate mouse-specific FAP x CD40 DARPin® molecule (mFAP x mCD40) was generated and tested in murine cell-based in vitro assays showing comparable results as MP0317 with a strict FAP-dependent activation of CD40. The mFAP x mCD40 was also active in vivo and inhibited substantially the progression of FAP-positive tumors. Moreover, in contrast to an anti-mouse CD40 antibody (clone FGK45), the antitumor activity of mFAP x mCD40 was neither associated with elevated blood cytokine levels nor with hepatotoxicity, both of which manifest as dose limiting toxicities of some of the clinical CD40 activating antibodies. These data support a mode of action, both in vitro and in vivo, that is dependent on FAP-mediated crosslinking of CD40 receptor resulting in a tumor-localized CD40 activation without peripheral or off-tumor organ toxicity.

In conclusion, we have designed a tumor-targeted CD40 agonistic DARPin® therapeutic candidate that is able to activate the CD40 receptor locally in FAP-positive tumors producing impressive antitumor activity in the absence of systemic toxicity.

Citation Format: Nicolo Rigamonti, Sarah Jetzer, Niels Van der Velden, Omar Abdelmotaleb, Sophie Barsin, Jonas Schwestermann, Susanne Mangold, Yvonne Kaufmann, Ralph Bessey, Andrea Valeri, Heidi Poulet, Christof Zitt, Pamela A. Trail, Victor Levitsky, Niina Veitonmäki, Clara Metz. A tumor-targeted CD40 agonistic DARPin® molecule leading to antitumor activity with limited systemic toxicity [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1073.

Preclinical Activity of the Novel Anti-Prolactin Receptor (PRLR) Antibody-Drug Conjugate REGN2878-DM1 in PRLR-Positive Breast Cancers

The Prolactin Receptor (PRLR) is a type 1 cytokine receptor that is expressed in a subset of breast cancers and may contribute to its pathogenesis. It is relatively overexpressed in approximately 25% of human breast tumors while expressed at low levels in some normal human tissues including the mammary gland. We developed an anti-PRLR antibody-drug conjugate (ADC), to target PRLR-positive breast cancer. REGN2878-DM1 is comprised of a fully human high-affinity function-blocking anti-PRLR IgG1 antibody (REGN2878) conjugated via a noncleavable SMCC linker to the cytotoxic maytansine derivative DM1. Both unconjugated REGN2878 and conjugated REGN2878-DM1 block PRL-mediated activation in vitro and are rapidly internalized into lysosomes. REGN2878-DM1 induces potent cell-cycle arrest and cytotoxicity in PRLR-expressing tumor cell lines. In vivo, REGN2878-DM1 demonstrated significant antigen-specific antitumor activity against breast cancer xenograft models. In addition, REGN2878-DM1 showed additive activity when combined with the antiestrogen agent fulvestrant. These results illustrate promising antitumor activity against PRLR-positive breast cancer xenografts and support the evaluation of anti-PRLR ADCs as potential therapeutic agents in breast cancer. Mol Cancer Ther; 16(7); 1299-311. ©2017 AACR.

Abstract C126: PRLR ADC: A novel antibody drug-conjugate for the treatment of PRLR positive breast cancer

Breast cancer is a heterogeneous disease comprised of various subtypes based on pathology and molecular profiling. Expression of hormone receptors (HR) and HER2 biomarkers are important determinants of therapy choice, due to the established role of these proteins as drivers of disease. Prolactin Receptor (PRLR) is a type 1 cytokine receptor that is expressed on a subset of breast cancers and may contribute to pathogenesis. Functionally, activation of PRLR by the hormone ligand Prolactin (PRL) induces PRLR dimerization and signaling resulting in cell proliferation and differentiation. While PRLR is expressed at low levels in some normal human tissues including the mammary gland, it is relatively overexpressed in ∼25% of human breast tumors and importantly, is rapidly internalized upon binding of anti-PRLR antibodies.

We developed an anti-PRLR antibody-drug conjugate (ADC), PRLR ADC, to target PRLR positive breast cancer. PRLR ADC is comprised of a fully human high affinity function-blocking anti-PRLR IgG1 antibody conjugated via a non-cleavable SMCC linker to the cytotoxic maytansine derivative DM1. Both unconjugated anti-PRLR antibody and the PRLR ADC block PRL mediated activation in vitro and induce rapid internalization of the receptor into lysosomes. PRLR ADC induces potent cell cycle arrest and cytotoxicity in several PRLR-expressing cell lines.

The in vivo efficacy of PRLR ADC was explored in breast cancer cell line xenograft models expressing both endogenous PRLR (MCF7, T47D) or transfected receptor (MCF7/PRLR). Treatment of tumor bearing SCID (T47D) or NCr Nude (MCF7) animals was initiated approximately 15 days post implantation of cells where tumor volumes averaged 150-200 mm3. In both T47D and MCF7/PRLR xenograft models, where PRLR is expressed highly, single or multiple (once weekly x 3) doses of 2.5-15 mg/kg resulted in significant inhibition of tumor xenograft growth. In the MCF7 model that expresses low levels of PRLR, inhibition and regression of tumors was observed at 10 and 15 mg/kg dose levels. In all models, higher doses resulted in greater and more prolonged repression of tumor growth. Conjugation of DM1 to anti-PRLR antibody was required for efficacy, as unconjugated antibody had no effect on tumor growth. Anti-tumor efficacy of PRLR ADC was also assessed in NSG mice bearing breast cancer Patient Derived Xenograft (PDXs) tumors with moderate and heterogeneous expression of PRLR. Treatment was initiated 21 days after implantation of the PDX tumors where the average tumor volume was ∼500mm3. Anti-tumor efficacy was observed following 10 or 20 mg/kg PRLR ADC dosed once weekly x 4.

These studies demonstrate the promising anti-tumor activity of the PRLR ADC against PRLR positive breast cancers and support the continued development of this agent.

Citation Format: Marcus P. Kelly, Sandra Coetzee, Carlos Hickey, Sosina Makonnen, Frank Delfino, Julian Andreev, Arthur Kunz, Christopher D'Souza, Jason Giurleo, Thomas Nittoli, Pamela A. Trail, Nicholas Papadopoulos, Gavin Thurston, Jessica R. Kirshner. PRLR ADC: A novel antibody drug-conjugate for the treatment of PRLR positive breast cancer. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C126.

A Phase I First-in-Human Study of Nesvacumab (REGN910), a Fully Human Anti-Angiopoietin-2 (Ang2) Monoclonal Antibody, in Patients with Advanced Solid Tumors

PURPOSE

Nesvacumab (REGN910) is a fully human immunoglobulin G1 (IgG1) monoclonal antibody that specifically binds and inactivates the Tie2 receptor ligand Ang2 with high affinity, but shows no binding to Ang1. The main objectives of this trial were to determine the safety, tolerability, dose-limiting toxicities (DLT), and recommended phase II dose (RP2D) of nesvacumab.

EXPERIMENTAL DESIGN

Nesvacumab was administered intravenously every two weeks with dose escalations from 1 to 20 mg/kg in patients with advanced solid tumors.

RESULTS

A total of 47 patients were treated with nesvacumab. No patients in the dose escalation phase experienced DLTs, therefore a maximum tolerated dose (MTD) was not reached. The most common nesvacumab-related adverse events were fatigue (23.4%), peripheral edema (21.3%), decreased appetite, and diarrhea (each 10.6%; all grade ≤ 2). Nesvacumab was characterized by linear kinetics and had a terminal half-life of 6.35 to 9.66 days in a dose-independent manner. Best response by RECIST 1.1 in 43 evaluable patients included 1 partial response (adrenocortical carcinoma) of 24 weeks duration. Two patients with hepatocellular carcinoma had stable disease (SD) > 16 weeks, with tumor regression and >50% decrease in α-fetoprotein. Analyses of putative angiogenesis biomarkers in serum and tumor biopsies were uninformative for treatment duration.

CONCLUSIONS

Nesvacumab safety profile was acceptable at all dose levels tested. Preliminary antitumor activity was observed in patients with treatment-refractory advanced solid tumors. On the basis of cumulative safety, antitumor activity, pharmacokinetic and pharmacodynamic data, the 20 mg/kg dose was determined to be the RP2D.

Dual IGF-I/II-neutralizing antibody MEDI-573 potently inhibits IGF signaling and tumor growth

Insulin-like growth factors (IGF), IGF-I and IGF-II, are small polypeptides involved in regulating cell proliferation, survival, differentiation, and transformation. IGF activities are mediated through binding and activation of IGF-1R or insulin receptor isoform A (IR-A). The role of the IGF-1R pathway in promoting tumor growth and survival is well documented. Overexpression of IGF-II and IR-A is reported in multiple types of cancer and is proposed as a potential mechanism for cancer cells to develop resistance to IGF-1R-targeting therapy. MEDI-573 is a fully human antibody that neutralizes both IGF-I and IGF-II and inhibits IGF signaling through both the IGF-1R and IR-A pathways. Here, we show that MEDI-573 blocks the binding of IGF-I and IGF-II to IGF-1R or IR-A, leading to the inhibition of IGF-induced signaling pathways and cell proliferation. MEDI-573 significantly inhibited the in vivo growth of IGF-I- or IGF-II-driven tumors. Pharmacodynamic analysis demonstrated inhibition of IGF-1R phosphorylation in tumors in mice dosed with MEDI-573, indicating that the antitumor activity is mediated via inhibition of IGF-1R signaling pathways. Finally, MEDI-573 significantly decreased (18)F-fluorodeoxyglucose ((18)F-FDG) uptake in IGF-driven tumor models, highlighting the potential utility of (18)F-FDG-PET as a noninvasive pharmacodynamic readout for evaluating the use of MEDI-573 in the clinic. Taken together, these results demonstrate that the inhibition of IGF-I and IGF-II ligands by MEDI-573 results in potent antitumor activity and offers an effective approach to selectively target both the IGF-1R and IR-A signaling pathways.

Sorafenib (BAY 43-9006, Nexavar), a dual-action inhibitor that targets RAF/MEK/ERK pathway in tumor cells and tyrosine kinases VEGFR/PDGFR in tumor vasculature

Activating mutations in Ras and B-RAF were identified in several human cancers. In addition, several receptor tyrosine kinases, acting upstream of Ras, were found either mutated or overexpressed in human tumors. Because oncogenic activation of the Ras/RAF pathway may lead to a sustained proliferative signal resulting in tumor growth and progression, inhibition of this pathway represents an attractive approach for cancer drug discovery. A novel class of biaryl urea that inhibits C-RAF kinase was discovered using a combination of medicinal and combinatorial chemistry approaches. This effort culminated in the identification of the clinical candidate BAY 43-9006 (Sorafenib, Nexavar), which has recently been approved by the FDA for advanced renal cell carcinoma in phase III clinical trials. Sorafenib inhibited the kinase activity of both C-RAF and B-RAF (wild type and V600E mutant). It inhibited MEK and ERK phosphorylation in various cancer cell lines and tumor xenografts and exhibited potent oral antitumor activity in a broad spectrum of human tumor xenograft models. Further characterization of sorafenib revealed that this molecule was a multikinase inhibitor that targeted the vascular endothelial growth factor receptor family (VEGFR-2 and VEGFR-3) and platelet-derived growth factor receptor family (PDGFR-beta and Kit), which play key roles in tumor progression and angiogenesis. Thus, sorafenib may inhibit tumor growth by a dual mechanism, acting either directly on the tumor (through inhibition of Raf and Kit signaling) and/or on tumor angiogenesis (through inhibition of VEGFR and PDGFR signaling). In phase I and phase II clinical trials, sorafenib showed limited side effects and, more importantly, disease stabilization. This agent is currently being evaluated in phase III clinical trials in renal cell and hepatocellular carcinomas.

Antitumor activity of sorafenib in FLT3-driven leukemic cells

Activating internal tandem duplication (ITD) insertions in the juxtamembrane domain of the FLT3 tyrosine kinase are found in about one fourth of patients with acute myeloid leukemia and have been shown to be an independent negative prognostic factor for survival. We show that sorafenib (BAY 43-9006, Nexavar) potently inhibits FLT3 enzymatic and signaling activities. In HEK293 cells stably transfected with FLT3-WT or FLT3-ITD, sorafenib blocked basal and ligand dependent FLT3-mediated tyrosine autophosphorylation as well as extracellular signal-regulated kinase1/2 and Stat5 phosphorylation. In leukemia cell lines MV4-11 and EOL-1, sorafenib treatment resulted in decreased cell proliferation and inhibition of FLT3 signaling. The growth of the FLT3-independent RS4-11 cell line was only weakly inhibited by sorafenib. Cell cycle arrest and induction of apoptosis were observed upon treatment with sorafenib in MV4-11 and EOL-1 cells. The antitumor efficacy of sorafenib was evaluated against the MV4-11 leukemia grown subcutaneously in NCr nu/nu mice. Doses of 3 and 10 mg/kg administered orally for 14 days resulted in six and nine out of 10 animals with complete responses, respectively. The demonstration that sorafenib exhibits potent target inhibition and efficacy in FLT3-driven models suggests that this compound may have a therapeutic benefit for patients with FLT3-driven leukemias.

Sorafenib (BAY 43-9006) inhibits tumor growth and vascularization and induces tumor apoptosis and hypoxia in RCC xenograft models

PURPOSE

New research findings have revealed a key role for vascular endothelial growth factor (VEGF) in the stimulation of angiogenesis in clear cell renal carcinoma (RCC) which is a highly vascularized and treatment-resistant tumor. Sorafenib (BAY 43-9006, Nexavar) is a multi-kinase inhibitor which targets receptor tyrosine and serine/threonine kinases involved in tumor progression and tumor angiogenesis. The effect of sorafenib on tumor growth and tumor histology was assessed in both ectopic and orthotopic mouse models of RCC.

METHODS

Sorafenib was administered orally to mice bearing subcutaneous (SC, ectopic) or sub-renal capsule (SRC, orthotopic) tumors of murine (Renca) or human (786-O) RCC. Treatment efficacy was determined by measurements of tumor volume and tumor growth delay. In mechanism of action studies, using the 786-O and Renca RCC tumor models, the effect of sorafenib was assessed after dosing for 3 or 5 days in the SC models and 21 days in the SRC models. Inhibition of tumor angiogenesis was assessed by measuring level of CD31 and alpha-smooth muscle actin (alphaSMA) staining by immunohistochemistry (IHC). The effect of sorafenib on MAPK signaling, cell cycle progression and cell proliferation was also assessed by IHC by measuring levels of phospho-ERK, phospho-histone H3 and Ki-67 staining, respectively. The extent of tumor apoptosis was measured by terminal deoxynucleotidyl transferase-mediated nick-end labeling (TUNEL) assays. Finally, the effects of sorafenib on tumor hypoxia was assessed in 786-O SC model by injecting mice intravenously with pimonidazole hydrochloride 1 h before tumor collection and tumor sections were stained with a FITC-conjugated Hypoxyprobe antibody.

RESULTS

Sorafenib produced significant tumor growth inhibition (TGI) and a reduction in tumor vasculature of both ectopic and orthotopic Renca and 786-O tumors, at a dose as low as 15 mg/kg when administered daily. Inhibition of tumor vasculature was observed as early as 3 days post-treatment, and this inhibition of angiogenesis correlated with increased level of tumor apoptosis (TUNEL-positive) and central necrosis. Consistent with these results, a significant increase in tumor hypoxia was also observed 3 days post-treatment in 786-O SC model. However, no significant effect of sorafenib on phospho-ERK, phospho-histone H3 or Ki-67 levels in either RCC tumor model was observed.

CONCLUSION

Our results show the ability of sorafenib to potently inhibit the growth of both ectopically- and orthotopically-implanted Renca and 786-O tumors. The observed tumor growth inhibition and tumor stasis or stabilization correlated strongly with decreased tumor angiogenesis, which was due, at least in part, to inhibition of VEGF and PDGF-mediated endothelial cell and pericyte survival. Finally, sorafenib-mediated inhibition of tumor growth and angiogenesis occurred at concentrations equivalent to those achieved in patients in the clinic.

BAY 43-9006 Exhibits Broad Spectrum Oral Antitumor Activity and Targets the RAF/MEK/ERK Pathway and Receptor Tyrosine Kinases Involved in Tumor Progression and Angiogenesis

The RAS/RAF signaling pathway is an important mediator of tumor cell proliferation and angiogenesis. The novel bi-aryl urea BAY 43-9006 is a potent inhibitor of Raf-1, a member of the RAF/MEK/ERK signaling pathway. Additional characterization showed that BAY 43-9006 suppresses both wild-type and V599E mutant BRAF activity in vitro. In addition, BAY 43-9006 demonstrated significant activity against several receptor tyrosine kinases involved in neovascularization and tumor progression, including vascular endothelial growth factor receptor (VEGFR)-2, VEGFR-3, platelet-derived growth factor receptor beta, Flt-3, and c-KIT. In cellular mechanistic assays, BAY 43-9006 demonstrated inhibition of the mitogen-activated protein kinase pathway in colon, pancreatic, and breast tumor cell lines expressing mutant KRAS or wild-type or mutant BRAF, whereas non-small-cell lung cancer cell lines expressing mutant KRAS were insensitive to inhibition of the mitogen-activated protein kinase pathway by BAY 43-9006. Potent inhibition of VEGFR-2, platelet-derived growth factor receptor beta, and VEGFR-3 cellular receptor autophosphorylation was also observed for BAY 43-9006. Once daily oral dosing of BAY 43-9006 demonstrated broad-spectrum antitumor activity in colon, breast, and non-small-cell lung cancer xenograft models. Immunohistochemistry demonstrated a close association between inhibition of tumor growth and inhibition of the extracellular signal-regulated kinases (ERKs) 1/2 phosphorylation in two of three xenograft models examined, consistent with inhibition of the RAF/MEK/ERK pathway in some but not all models. Additional analyses of microvessel density and microvessel area in the same tumor sections using antimurine CD31 antibodies demonstrated significant inhibition of neovascularization in all three of the xenograft models. These data demonstrate that BAY 43-9006 is a novel dual action RAF kinase and VEGFR inhibitor that targets tumor cell proliferation and tumor angiogenesis.

BR96 conjugates of highly potent anthracyclines

The 6-maleimidocaproylhydrazone derivatives of highly potent antitumor agents 5-Diacetoxypentyldoxorubicin and Morpholinodoxorubicin were synthesized and conjugated to monoclonal antibody BR96 and control IgG. Immunoconjugate molar ratios were generally 7.5-8.5, and dimer aggregate levels were low. The linkers released parent drug at lysosomal pH 5, while they remained stable at neutral pH. BR96 conjugates were highly potent and antigen specific in vitro. The BR96-DAPDOX conjugate demonstrated an IC(50) of 0.03 micrometer and was at least 300-fold more potent than a non-binding IgG-DAPDOX control conjugate.

Monoclonal antibody drug immunoconjugates for targeted treatment of cancer

Monoclonal antibodies (mAb) directed to tumor-associated antigens (TAA) or antigens differentially expressed on the tumor vasculature have been covalently linked to drugs that have different mechanisms of action and various levels of potency. The use of these mAb immunoconjugates to selectively deliver drugs to tumors has the potential to both improve antitumor efficacy and reduce the systemic toxicity of therapy. Several immunoconjugates, particularly those that incorporate internalizing antibodies and tumor-selective linkers, have demonstrated impressive activity in preclinical models. Immunoconjugates that deliver doxorubicin, maytansine and calicheamicin are currently being evaluated in clinical trials. The feasibility of using immunoconjugates as cancer therapeutics has been clearly demonstrated. Gemtuzumab ozogamicin, a calicheamicin conjugate that targets CD33, has recently been approved by the Food and Drug Administration (FDA) for treatment of acute myelogenous leukemia (AML). This review concentrates on the properties of the tumor and the characteristics of the mAb, linker, and drugs that influence the efficacy, potency, and selectivity of immunconjugates selected for cancer treatment.

Monoclonal antibody conjugates of doxorubicin prepared with branched peptide linkers: inhibition of aggregation by methoxytriethyleneglycol chains

High mole ratio BR96 immunoconjugates were synthesized using branched peptide-doxorubicin linkers designed to liberate doxorubicin following antigen-specific internalization into lysosomes. However, these immunoconjugates are highly prone to noncovalent, dimeric aggregation. We hypothesize that this is due to (1) the hydrophobic nature of the peptides, (2) the loss of positive charge upon amide formation at the 3'-amino group of doxorubicin, and (3) the proximity of the peptide hydrophobic residues to form efficient intermolecular stacking interactions. By introducing a hydrophilic methoxytriethylene glycol chain onto the doxorubicin portion of the branched peptide linkers, aggregation has been eliminated or greatly reduced in the immunoconjugate products. The methoxytriethylene glycol chain was linked to the doxorubicin moiety of the linker via a hydrazone bond that is stable at pH 7 but hydrolyzes rapidly at pH 5 to release free drug. BR96 immunoconjugates synthesized from methoxytriethylene glycol-modified branched peptide-doxorubicin linkers are highly potent and immunospecific in vitro. The data suggest that the methoxytriethylene glycol chain hydrolyzes as designed upon antigen-specific internalization into tumor lysosomes in vitro, where enzymatic degradation of the peptide linker releases free doxorubicin.

Cathepsin B-labile dipeptide linkers for lysosomal release of doxorubicin from internalizing immunoconjugates: model studies of enzymatic drug release and antigen-specific in vitro anticancer activity

The anticancer drug doxorubicin (DOX) has been linked to chimeric BR96, an internalizing monoclonal antibody that binds to a Lewis(y)-related, tumor-associated antigen, through two lysosomally cleavable dipeptides, Phe-Lys and Val-Cit, giving immunoconjugates 72 and 73. A self-immolative p-aminobenzyloxycarbonyl (PABC) spacer between the dipeptides and the DOX was required for rapid and quantitative generation of free drug. DOX release from model substrate Z-Phe-Lys-PABC-DOX 49 was 30-fold faster than from Z-Val-Cit-PABC-DOX 42 with the cysteine protease cathepsin B alone, but rates were identical in a rat liver lysosomal preparation suggesting the participation of more than one enzyme. Conjugates 72 and 73 showed rapid and near quantitative drug release with cathepsin B and in a lysosomal preparation, while demonstrating excellent stability in human plasma. Against tumor cell lines with varying levels of BR96 expression, both conjugates showed potent, antigen-specific cytotoxic activity, suggesting that they will be effective in delivering DOX selectively to antigen-expressing carcinomas.

Doxorubicin immunoconjugates containing bivalent, lysosomally-cleavable dipeptide linkages

Bivalent doxorubicin (DOX)-dipeptides (16a-c) were prepared and conjugated to the monoclonal antibody BR96. The dipeptides are cleaved by lysosomal proteases following internalization of the resulting immunoconjugates. Conjugate 18b demonstrated antigen-specific in vitro tumor cell killing activity (IC(50)=0.2 microM) that was equipotent to DOX with a near doubling of drug molecules/MAb. Size exclusion chromatography showed 18b to be a noncovalent dimer that was formed immediately upon conjugation.

Synthesis of an immunoconjugate of camptothecin

The first immunoconjugate of camptothecin has been synthesized wherein the drug is attached to the tumor-recognizing antibody BR96 via a Cathepsin B cleavable linker. Endocytosis of the immunoconjugate upon binding to the tumor cell followed by enzymatic cleavage of the linker inside the endosome ensures tumor-specific release of the drug. In this way, it is hoped that the dose-limiting side effects associated with camptothecin can be eliminated while the antitumor activity is preserved.

Inhibition of angiogenesis and metastasis in two murine models by the matrix metalloproteinase inhibitor, BMS-275291

BMS-275291 is an p.o. bioavailable, sulfhydryl-based matrix metalloproteinase (MMP) inhibitor currently in clinical development for the treatment of cancer. This inhibitor was designed to potently inhibit MMP activities while minimally affecting those of other metalloproteases (e.g., sheddases) involved in the release of cell-associated molecules such as tumor necrosis factor-alpha, tumor necrosis factor-alpha receptor, interleukin-6 receptor, or L-selectin. In vitro, BMS-275291 is a potent inhibitor (nM) of the activities of MMP-1, MMP-2, MMP-7, MMP-9, and MMP-14. BMS-275291 inhibits tumor growth in a B16BL6 model of experimental metastasis, and in this model, BMS-275291 treatment results in a dose-dependent reduction in the number of lung metastases compared with vehicle controls. BMS-275291 also inhibits angiogenesis in a murine angiogenesis model, where once daily treatment with BMS-275291 results in a dose-dependent inhibition of endothelial cell migration into s.c. implanted Matrigel plugs. Pharmacokinetic studies demonstrated that the plasma concentrations of parent BMS-275291 in mice exceeds the in vitro IC(50) values for MMP-1, MMP-2, MMP-7, MMP-9, and MMP-14 for at least 4 h after the administration of a therapeutic dose of BMS-275291. Taken together, these data demonstrate that BMS-275291 inhibits MMP activities that contribute to tumor metastasis and angiogenesis.

Selective tumor sensitization to taxanes with the mAb-drug conjugate cBR96-doxorubicin

The chimeric monoclonal antibody cBR96 conjugated to doxorubicin (cBR96-Dox) is selectively internalized by a wide variety of human carcinomas expressing an extended form of Lewis Y antigen (Le(y)). Endocytosis is followed by cleavage and release of free doxorubicin from the endocytic vesicles and subsequent cytotoxicity. Combination studies with standard anti-cancer agents, undertaken to further increase the potency of this targeted therapy, identified significant synergistic anti-tumor activity of cBR96-Dox and either of the taxanes paclitaxel or docetaxel. Treatment with cBR96-Dox 24 hr prior to paclitaxel resulted in a steady increase in the percentage of G(2) tumor cells and corresponding increase in sensitivity to taxanes. Cell cycle analysis indicated the cBR96-delivered doxorubicin was most effective against S-phase cells, yet cells exposed to even subtoxic levels progressed to and arrested in G(2), at a point of high sensitivity to the anti-tubulin agent paclitaxel. The synergy obtained by staged combination of cBR96-Dox and paclitaxel in vitro was reflected in significant anti-tumor efficacy in vivo against xenograft models of human lung and breast tumors that could not be achieved by either agent alone. The staged combination elicited significant or complete regressions of established human Le(y)-positive tumor xenografts using significantly reduced drug levels. Taken together, these data demonstrate a mechanistic approach to the selective elimination of Le(y)-positive tumors by using targeted doxorubicin followed by taxane treatment.

Constitutive expression of functional 4-1BB (CD137) ligand on carcinoma cells

Members of the TNF superfamily, including Fas, Fas ligand, and CD40, have been shown to be expressed on tumor cells. In the studies described in this work, we report that another family member, the ligand for 4-1BB (CD137), is expressed on various human carcinoma cell lines, on cells of solid tumors derived from these cell lines, and cells obtained from human tumors. Expression of 4-1BB ligand (4-1BBL) mRNA was detected by both RT-PCR and Northern blot analysis, and expression of 4-1BBL protein was detected by Western blot analysis of whole cell lysates and by FACS analysis of tumor cells and cell lines. Incubation of tumor cells with a 4-1BB-Ig fusion protein led to the production of IL-8 by the cells, demonstrating that the 4-1BBL is functionally active and signals back into the tumor cells. Furthermore, 4-1BBL expressed on the carcinoma cells functioned as a costimulatory molecule for the production of cytokines (most notably IFN-gamma) in cocultures of T cells and tumor cells. These findings suggest that 4-1BBL expressed on carcinoma cells may significantly influence the outcome of a T cell-tumor cell interaction.

Enhanced delivery improves the efficacy of a tumor-specific doxorubicin immunoconjugate in a human brain tumor xenograft model

OBJECTIVE

To evaluate dose intensification with osmotic blood-brain barrier disruption (BBBD) and the potential use of drug targeting with monoclonal antibody (MAb) BR96 conjugated to doxorubicin (BR96-DOX, now called SGN15) for treatment of intracerebral and subcutaneous human LX-1 small cell lung carcinoma xenografts in rats.

METHODS

LX-1 tumors with high, low, or heterogeneous levels of the Lewis(y) antigen for BR96 were evaluated. Rats were treated with intracarotid or intravenous BR96-DOX, with or without osmotic BBBD.

RESULTS

Both BR96-DOX and MAb BR96 treatment resulted in significant regression of subcutaneous tumors, in contrast to control groups including doxorubicin alone, saline, or nonbinding doxorubicin immunoconjugate. BR96-DOX delivered with BBBD to brain tumors with low antigen expression resulted in significantly (P < 0.001) increased rat survival time compared with animals that received intravenous or intra-arterial BR96-DOX.

CONCLUSION

The combination of an effective drug such as doxorubicin with a MAb to facilitate tumor-selective localization and osmotic BBBD to increase tumor delivery may have practical application in the clinic, because an increased delivery of drug to tumor can be obtained without increasing the dose of systemic drug.

Enhanced antitumor activity of paclitaxel in combination with the anticarcinoma immunoconjugate BR96-doxorubicin

The efficacy of chemotherapy has been improved by regimens that combine several cytotoxic drugs with different mechanisms of action and/or different dose-limiting toxicities. Here we demonstrate clearly, and for the first time, that combined therapy using an anticarcinoma immunoconjugate, BR96-doxorubicin, and the cytotoxic drug paclitaxel results in a significant increase in antitumor activity over that of either agent alone. Synergistic activity was seen at doses of BR96-doxorubicin that were minimally active as single agents. A dramatic increase in regression rates was seen when a regimen that combined BR96-doxorubicin and paclitaxel was used to treat both paclitaxel-sensitive and paclitaxel-insensitive carcinomas. Importantly, combined therapy resulted in increased antitumor activity against lung, colon, and breast tumors xenografted in athymic mice and large, paclitaxel-insensitive colon tumors xenografted in athymic rats that also express the Lewis(y) target antigen in normal tissues.

Monoclonal antibody drug conjugates in the treatment of cancer

Monoclonal antibodies directed to tumor-associated antigens have been chemically conjugated to drugs with different mechanisms of action and different levels of potency. Monoclonal-antibody-directed drug delivery has the potential to both improve efficacy and reduce systemic toxicity. Several immunoconjugates have demonstrated impressive antigen-specific antitumor activity in preclinical models. Phase I trials of a calicheamicin immunoconjugate for treatment of acute myeloid leukemia and a doxorubicin immunoconjugate for treatment of carcinoma have recently been completed.

Monoclonal antibody conjugates of doxorubicin prepared with branched linkers: A novel method for increasing the potency of doxorubicin immunoconjugates

Immunoconjugates of monoclonal antibody BR96 and Doxorubicin have been prepared using a novel series of branched hydrazone linkers. Since each linker bound to the mAb carries two DOX molecules, the DOX/mAb molar ratios of these conjugates were approximately 16, twice that of those previously prepared with single-chain hydrazone linkers. The conjugates were stable at a physiological pH of 7, but released DOX rapidly at lysosomal pH 5. The branched series of BR96 conjugates demonstrated antigen-specific cytotoxicity, and were more potent in vitro than the single-chain conjugate on both a DOX (4-14-fold) and mAb (7-23-fold) basis. The results suggest that, by using the branched linker methodology, it is possible to significantly reduce the amount of mAb required to achieve antigen-specific cytotoxic activity. In this paper, the synthesis and in vitro biology of branched chain immunoconjugates are described.

Characterization of kringle domains of angiostatin as antagonists of endothelial cell migration, an important process in angiogenesis

Angiogenesis is a complex process that involves endothelial cell proliferation, migration, basement membrane degradation, and neovessel organization. Angiostatin, consisting of four homologous triple-disulfide bridged kringle domains, has previously been shown to exhibit profound inhibition of endothelial cell proliferation in vitro and angiogenesis in vivo. It was also demonstrated that angiostatin could suppress the growth of a variety of tumors via the blocking of angiogenesis. The primary aim of our study was to characterize the kringle domains of angiostatin for their inhibitory activities of endothelial cell migration in order to elucidate their contributions to the anti-angiogenic function of angiostatin. In this report, we demonstrate for the first time that the kringles of angiostatin play different roles in inhibiting endothelial cell migration, a crucial process in angiogenesis. Kringle 4, which has only marginal anti-proliferative activity, is among the most potent fragments in inhibiting endothelial cell migration (IC50 of approximately 500 nM). In contrast, kringle 1-3, which is equivalent to angiostatin in inhibiting endothelial cell proliferation, manifests only a modest anti-migratory effect. The combination of kringle 1-3 and kringle 4 results in an anti-migratory activity comparable to that of angiostatin. When kringle 1 is removed from kringle 1-3, the resulting kringle 2-3 becomes more potent than kringle 1-3. This implies that kringle 1, although virtually ineffective in inhibiting endothelial cell migration, may influence the conformation of kringle 1-3 to alter its anti-migratory activity. We also show that disruption of the kringle structure by reducing/alkylating agents markedly attenuates the anti-migratory activity of angiostatin, demonstrating the significance of kringle conformation in maintaining the anti-angiogenic activity of angiostatin. Our data suggest that different kringle domains may contribute to the overall anti-angiogenic function of angiostatin by their distinct anti-migratory activities.

Selective inhibition by kringle 5 of human plasminogen on endothelial cell migration, an important process in angiogenesis

Angiogenesis is a multi-step process that includes endothelial cell proliferation, migration, basement membrane degradation, and new lumen organization. Angiostatin, an internal fragment of plasminogen comprising the first four triple disulfide-linked kringle structures, is one of the most potent endogenous angiogenesis inhibitors described to date. The kringle 5 domain of plasminogen, which shares high sequence homology with the four kringles of angiostatin, was previously shown to antagonize endothelial cell growth. We now describe that the recombinant kringle 5 of human plasminogen inhibits endothelial cell migration with an IC50 (concentration for half maximal inhibition) of approximately 500 nM. We demonstrate that the lysine-binding sites of kringle 5 may not be involved in its anti-migratory activities. The anti-migratory activity of kringle 5 is similar to that of angiostatin. Kringle 5 also shows selective inhibition on endothelial cells as opposed to other cell types. Relative to its native form, reduced kringle 5 displays a significant increase in anti-migratory activity, implying that the kringle conformation may shield kringle 5 from effectively interacting with endothelial cells. This report thus constitutes the first demonstration that kringle 5 of plasminogen is a selective inhibitor for endothelial cell migration.

Antitumor activity of carcinoma-reactive BR96-doxorubicin conjugate against human carcinomas in athymic mice and rats and syngeneic rat carcinomas in immunocompetent rats

The internalizing monoclonal antibody BR96 was conjugated to the anticancer drug doxorubicin (DOX) using an acid-labile hydrazone bond to DOX and a thioether bond to the monoclonal antibody. The resulting conjugate, termed BR96-DOX, binds to a tumor-associated Lewis(y) antigen that is abundantly expressed on the surface of human carcinoma cells. BR96-DOX binds to RCA, a human colon carcinoma cell line, and BN7005, a transplantable colon carcinoma induced in a Brown Norway (BN) rat by 1,2-dimethyl-hydrazine. BR96-DOX produces cures of established s.c. RCA human colon carcinomas in athymic mice and rats. BR96-DOX also cured both s.c. and intrahepatic BN7005 tumors in immunocompetent BN rats. Unconjugated DOX, given at its maximum tolerated dose, and matching doses of nonbinding IgG-DOX conjugate were not active against RCA or BN7005 carcinomas. An anticonjugate antibody response was produced in BN rats treated with BR96-DOX. However, this could be largely prevented by administering the immunosuppressive drug deoxyspergualin. These results confirm the concept of antibody-directed therapy in models in which the targeted antigen is expressed both in normal tissues and tumors. The findings in BN7005 further demonstrate efficacy of BR96-DOX therapy in a model in which the tumor is syngeneic and the host is immunocompetent.

Effect of linker variation on the stability, potency, and efficacy of carcinoma-reactive BR64-doxorubicin immunoconjugates

The internalizing anti-Le(y) monoclonal antibody (MAb) BR64 was conjugated to the anticancer drug doxorubicin (DOX) using an acid-labile hydrazone bond to the DOX and either a disulfide or thioether bond to the MAb. The resulting disulfide (BR64-SS-DOX) and thioether (BR64-S-DOX) conjugates were evaluated for stability, potency, and antigen-specific activity in both in vitro and in vivo model systems. The BR64-SS-DOX conjugates demonstrated antigen-specific activity both in vitro and when evaluated against antigen-expressing, DOX-sensitive human carcinoma xenografts. However, the stability and potency of disulfide conjugates were poor, and in vivo activity superior to unconjugated DOX was seen only at doses approaching the maximum tolerated dose. Furthermore, BR64-SS-DOX conjugates were not active against antigen-expressing, DOX-insensitive colon tumor xenografts. In contrast, the BR64-S-DOX conjugates demonstrated good stability both in vitro and in vivo. The increased stability of the BR64-S-DOX conjugates resulted in the delivery of more biologically active DOX to tumors with a concomitant increase in potency and efficacy over that which could be achieved with either unconjugated DOX or BR64-SS-DOX conjugates. Delivery of DOX by BR64-SS-DOX conjugates resulted in complete regressions and cures of both DOX-sensitive lung xenografts and DOX-intensitive colon tumor xenografts. These results demonstrate the importance of linker stability when delivering drugs such as DOX to carcinomas via internalizing antibodies and are likely to have direct relevance to the clinical utility of MAb-directed delivery.

Reversal of the human and murine multidrug-resistance phenotype with megestrol acetate

MA is an orally active PG derivative with an excellent safety profile that is used primarily for the treatment of carcinomas of the breast and endometrium. We investigated the potential application of MA as an MDR-reversal agent using cell culture and human tumor xenograft models. The reversing activity of MA in vitro was compared with that of PG and VER in two human MDR cell lines, the colon carcinoma HCT-116/VM46 and the breast carcinoma MCF-7/ADR, and in a murine cell line, J774.2. At concentrations as low as 3 microM, MA was capable of partially restoring sensitivity to Act D in the HCT-116/VM46 cells and sensitivity to DOX in the MCF-7/ADR cells. Although less effective than VER, MA was about 2.5 times more potent than PG in reversing MDR at equimolar concentrations. Increased accumulation of DOX in drug-resistant cells that were treated simultaneously with MA was observed by flow cytometry. In vivo, using established human colon and breast carcinoma xenografts implanted s.c. in athymic mice, the combined therapy with MA and DOX resulted in enhanced antitumor activity relative to that of DOX alone in the MDR sublines. These results suggest that MA may be a promising clinical MDR-reversing agent.

(6-Maleimidocaproyl)hydrazone of doxorubicin--a new derivative for the preparation of immunoconjugates of doxorubicin

The (6-maleimidocaproyl)hydrazone of doxorubicin was synthesized and conjugated to several mAbs, including chimeric BR96, via a Michael addition reaction to thiol-containing mAbs. DTT reduction of disulfides present in the mAb was a reliable and general method for generating a consistent number of reactive SH groups. The conjugates, after purification by Bio-Beads, were free of unreacted linker and/or doxorubicin. All conjugates released doxorubicin under acidic conditions that mimic the lysosomal environment, while they were relatively stable at neutral pH. BR96 conjugates showed antigen-specific cytotoxicity.

Cure of xenografted human carcinomas by BR96-doxorubicin immunoconjugates

Immunoconjugates (BR96-DOX) were prepared between chimeric monoclonal antibody BR96 and the anticancer drug doxorubicin. The monoclonal antibody binds an antigen related to Lewis Y that is abundantly expressed at the surface of cells from many human carcinomas; it has a high degree of tumor selectivity and is internalized after binding. BR96-DOX induced complete regressions and cures of xenografted human lung, breast, and colon carcinomas growing subcutaneously in athymic mice and cured 70 percent of mice bearing extensive metastases of a human lung carcinoma. Also, BR96-DOX cured 94 percent of athymic rats with subcutaneous human lung carcinoma, even though the rats, like humans and in contrast to mice, expressed the BR96 target antigen in normal tissues.

Antitumor activity of the single-chain immunotoxin BR96 sFv-PE40 against established breast and lung tumor xenografts

We have constructed a single-chain immunotoxin consisting of the variable H and L chains of the carcinoma-reactive mAb BR96, fused to the binding defective protein toxin, PE40. This molecule, BR96 sFv-PE40, has been shown to be extremely cytotoxic toward a variety of BR96 Ag-expressing tumor cell lines. When administered i.v. into athymic mice carrying L2987 tumor xenografts, BR96 sFv-PE40 was cleared rapidly from the blood with a half-life of approximately 30 min. This is in comparison to a chemical conjugate, chiBR96-LysPE40, that remained in the blood for almost 2 h. In addition, the smaller single-chain immunotoxin (67 kDa) penetrates the tumor faster than the larger chemical conjugate (190 kDa). Using a variety of administration schedules and doses, we treated established human tumor xenografts in athymic mice with both the single-chain immunotoxin BR96 sFv-PE40 and the chemical conjugate chiBR96-LysPE40. In both L2987 lung carcinoma and MCF-7 breast carcinoma models, we found that BR96 sFv-PE40 completely regressed the tumor xenografts. With an administration schedule of q4dx5, the tumors were totally regressed and did not reappear. The chiBR96-LysPE40 conjugate produced partial tumor regressions, although at near maximum tolerated dose. These results show that the single-chain immunotoxin, BR96 sFv-PE40, is a potent antitumor agent.

Antitumor activity of the single-chain immunotoxin BR96 sFv-PE40 against established breast and lung tumor xenografts

We have constructed a single-chain immunotoxin consisting of the variable H and L chains of the carcinoma-reactive mAb BR96, fused to the binding defective protein toxin, PE40. This molecule, BR96 sFv-PE40, has been shown to be extremely cytotoxic toward a variety of BR96 Ag-expressing tumor cell lines. When administered i.v. into athymic mice carrying L2987 tumor xenografts, BR96 sFv-PE40 was cleared rapidly from the blood with a half-life of approximately 30 min. This is in comparison to a chemical conjugate, chiBR96-LysPE40, that remained in the blood for almost 2 h. In addition, the smaller single-chain immunotoxin (67 kDa) penetrates the tumor faster than the larger chemical conjugate (190 kDa). Using a variety of administration schedules and doses, we treated established human tumor xenografts in athymic mice with both the single-chain immunotoxin BR96 sFv-PE40 and the chemical conjugate chiBR96-LysPE40. In both L2987 lung carcinoma and MCF-7 breast carcinoma models, we found that BR96 sFv-PE40 completely regressed the tumor xenografts. With an administration schedule of q4dx5, the tumors were totally regressed and did not reappear. The chiBR96-LysPE40 conjugate produced partial tumor regressions, although at near maximum tolerated dose. These results show that the single-chain immunotoxin, BR96 sFv-PE40, is a potent antitumor agent.

BR96 sFv-PE40, a potent single-chain immunotoxin that selectively kills carcinoma cells

We have constructed a single-chain immunotoxin composed of the carcinoma-reactive antibody BR96 and a truncated form of Pseudomonas exotoxin. The chimeric molecule, BR96 sFv-PE40, was expressed in Escherichia coli and localized to the inclusion bodies. We purified and identified two species of BR96 sFv-PE40, monomers and aggregates. The monomeric form was able to bind well to the BR96 antigen, a Lewisy-related antigen, while the aggregate was not. The binding affinity of the monomeric recombinant immunotoxin was 5-fold less than intact BR96 IgG, and its specificity for the BR96 antigen was confirmed by competition analysis. Monomeric BR96 sFv-PE40 was found to be extremely cytotoxic against cancer cells displaying the BR96 antigen. The cytotoxicity of the fusion protein correlates directly with antigen density on the tumor cell lines tested. The breast carcinoma cell line MCF-7, which has the highest density of BR96 antigen, was the most sensitive to BR96 sFv-PE40, with a concentration producing 50% protein synthesis inhibition of 5 pM. BR96 sFv-PE40 was found to have a t1/2 in serum of 28.5 min in athymic mice, compared to that of the chemical conjugate, chiBR96-LysPE40, which was 54 min. These data indicate that the single-chain immunotoxin BR96 sFv-PE40 is a potent inhibitor of protein synthesis in target cell lines and may be an effective agent for the treatment of cancer.

Antigen-specific activity of carcinoma-reactive BR64-doxorubicin conjugates evaluated in vitro and in human tumor xenograft models

The anticarcinoma antibody BR64 was conjugated to a doxorubicin derivative, doxorubicin 13-[3-(2-pyridyldithio)propionyl]hydrazone, and the resulting conjugates (BR64-DOX) were evaluated for activity and immunological specificity in vitro and in human tumor xenograft models. The BR64-DOX immunoconjugates retained immunoreactivity and cytotoxicity and demonstrated antigen-specific cytotoxicity in vitro. The potency of BR64-DOX immunoconjugates in vitro was related to the drug:monoclonal antibody mole ratio of the conjugates. The antitumor activity of BR64-DOX conjugates was consistently superior to the maximal activity obtained with the parent drug, doxorubicin (DOX), in established human lung and human breast carcinoma xenograft models. The superior antitumor activity of BR64-DOX conjugates was reflected both in tumor growth inhibition and in regressions and cures of established tumors following the administration of tolerated doses of BR64-DOX. The antitumor activity of BR64-DOX conjugates was not the result of synergism between monoclonal antibody BR64 and DOX, because mixtures consisting of monoclonal antibody and optimized DOX were not more active than an equivalent dose of DOX administered alone. The antitumor activity of BR64-DOX conjugates was antigen specific; equivalent doses of nonbinding isotype-matched conjugates were not active against established tumor xenografts.

Preclinical antitumor activity of a soluble etoposide analog, BMY-40481-30

BMY-40481-30 is a new, water-soluble derivative and probable prodrug of etoposide characterized by the presence of a phosphate group in position 4' of the E ring of the etoposide molecule. The compound was only weakly cytotoxic in vitro and, consequently, an investigation of its antitumor activity was conducted in several murine and human tumor (xenograft) models. Etoposide was administered ip or po whereas BMY-40481-30 was given ip, po or iv. The potency of the derivative, when administered parenterally, as defined on the basis of maximum tolerated dose (MTD), was less than the parent compound on a weight (mg/kg) basis in some experiments but comparable to etoposide in other instances. Comparison at the MTD of the two compounds showed that BMY-40481-30 administered ip was as active as etoposide against ip P388 leukemia. BMY-40481-30 given iv was more active than etoposide given ip in two of five experiments versus iv P388 leukemia, but the two compounds were comparably active in the other three studies. Of particular interest was the finding that the derivative was more active than the parent compound at many of the comparable (on a mg/kg basis) dose levels of both evaluated po versus iv P388 leukemia; MTD levels were not achieved, and hence not compared, for either compound using the po route of administration. Both etoposide and BMY-40481-30 yielded comparable maximum effects against ic P388 leukemia, ic L1210 leukemia, and sc B16 melanoma, but etoposide was more efficacious versus sc M5076 sarcoma.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of a human xenograft model for the evaluation of monoclonal antibody L6-mitomycin immunoconjugates

The human lung carcinoma H2981, was characterized as a preclinical model for evaluating immunoconjugates consisting of monoclonal antibody (MAb) L6 and drugs of the mitomycin (MMC) chemotype. The H2981 tumor, implanted subcutaneously in athymic, mice grew progressively in greater than 95% of recipients. Spontaneous regressions of established tumors were not observed. The administration of tolerated doses of MMC resulted in dose-dependent delays in tumor growth. The incidence of tumor regressions was low indicating the potential to observe increased efficacy with immunoconjugates. The antitumor effects of MMC were independent of the schedule and route of administration. MAb L6 delayed the outgrowth of tumors when administered 24 h after tumor implant. Antitumor activity was not observed when MAb L6 was administered to mice bearing established tumors. The efficacy of mixtures of MAb L6 and optimal doses of MMC was not significantly better than that of optimal doses of MMC given alone.

Canine cyclic hematopoiesis: alterations in T lymphocyte subpopulations in peripheral blood, lymph nodes, and thymus of gray collie dogs

Cyclic hematopoiesis (CH), also called cyclic neutropenia, is an inherited disorder known to occur in both humans and gray collie dogs. Previous reports have provided ample evidence to suggest that lymphocyte activity and regulatory mechanisms may be abnormal in CH. The present study examined the lymphocyte populations of several lymphoid compartments of gray collie dogs. The percentage of B lymphocytes in the lymph nodes of CH dogs was significantly increased whereas that of null lymphocytes was decreased. The percentage of T lymphocytes did not differ between CH and normal dogs, however, the proportions of T lymphocyte subpopulations were significantly different. The levels of T lymphocytes expressing IgGFc receptors (T gamma) in the thymus, lymph nodes, and peripheral blood were significantly increased; whereas the levels of T lymphocytes expressing IgMFc receptors (T mu) were significantly decreased. The percentage and absolute numbers of T gamma and T mu lymphocytes cycled in CH dogs. The percentage and absolute numbers of neutrophils were greatest when that of T gamma lymphocytes was reduced. The cycles of monocytes and T gamma lymphocytes occurred in close association and a linear relationship between the levels of these cells was observed both in terms of percentage (r = 0.62; P less than 0.01) and absolute number (r = 0.67; P less than 0.05). The percentage of T gamma and T mu lymphocytes were inversely correlated (r = -0.68; P less than 0.01).

Differential expression of lymphokine-activated killer cells and natural killer cells in adoptive transfer experiments utilizing fractionated bone marrow

Precursors of murine natural killer (NK) cells and lymphokine-activated killer (LAK) cells can be distinguished by utilizing an adoptive transfer system in which donor bone marrow is fractionated on Percoll discontinuous gradients. Although precursors of LAK cells are present in all fractions, one fraction (greater than 65% Percoll) contains LAK precursors and is depleted of NK precursors. Both in vitro NK activity and in vivo hybrid resistance is abrogated in recipients of bone marrow from the greater than 65% Percoll fraction, whereas LAK activity can be readily demonstrated.

Differential expression of lymphokine-activated killer cells and natural killer cells in adoptive transfer experiments utilizing fractionated bone marrow

Precursors of murine natural killer (NK) cells and lymphokine-activated killer (LAK) cells can be distinguished by utilizing an adoptive transfer system in which donor bone marrow is fractionated on Percoll discontinuous gradients. Although precursors of LAK cells are present in all fractions, one fraction (greater than 65% Percoll) contains LAK precursors and is depleted of NK precursors. Both in vitro NK activity and in vivo hybrid resistance is abrogated in recipients of bone marrow from the greater than 65% Percoll fraction, whereas LAK activity can be readily demonstrated.

Canine transmissible venereal sarcoma: quantitation of T-lymphocyte subpopulations during progressive growth and spontaneous tumor regression

The levels of T-lymphocyte subpopulations expressing IgMFc and IgGFc receptors (i.e., T mu lymphocytes and T gamma lymphocytes, respectively) were quantitated for T-lymphocytes obtained from peripheral blood, draining and nondraining lymph nodes, and the tumor mass during progressive growth and spontaneous regression of the canine transmissible venereal sarcoma. Analysis of the T-lymphocyte subpopulations in these lymphoid compartments demonstrated that distinct profiles of T mu and T gamma lymphocytes correlated respectively with the growth and regression statuses of the tumor. The percent of T gamma lymphocytes in the peripheral blood of dogs with progressing, steady-state, and late-regressing tumors was significantly increased over that of control dogs (P less than .05, P less than .001, and P less than .001, respectively) and over that of dogs with early-regressing tumors (P less than .05, P less than .01, and P less than .02, respectively). The percent of T gamma lymphocytes in the lymph nodes draining the tumor was observed to have a significant increase in dogs with progressing (P less than .01), steady-state (P less than .01), and late-regressing (P less than .001) tumors compared with that in control dogs. The percentage of T gamma lymphocytes was observed to have a significant increase in the nondraining lymph nodes of dogs with steady-state and late-regressing tumors compared with that of control dogs (P less than .01 and P less than .002, respectively) and that of dogs with progressing tumors (P less than .001 and P less than .0005, respectively). The percent of tumor-infiltrating T gamma lymphocytes was lowest in tumors that were growing progressively. A significant increase in T gamma lymphocytes was observed in steady-state (P less than .05), early-regressing (P less than .001), and late-regressing (P less than .05) tumors. Early-regressing tumors contained significantly (P less than .005) greater levels of T gamma lymphocytes than did late-regressing tumors.

Increase in the haemolytic complement activity of dogs affected with cyclic haematopoiesis

Cyclic haematopoiesis (CH) is an inherited disorder which occurs in both humans and Grey Collie dogs. The disease is characterized by fluctuations in the numbers of peripheral blood leucocytes, reticulocytes and platelets at regular intervals and is associated with a variety of clinical signs. The most prominent cycle observed is that of neutrophils. The 12-day neutropenic cycle includes a period of relatively normal neutrophil counts, a period of neutropenia and generally a period in which neutrophil counts greatly exceed the normal range. In this study the daily serum haemolytic complement activity (classical pathway CH50) of CH and normal Collie dogs was assayed. The serum CH50 of normal Collie dogs was relatively stable throughout the test period. In contrast, the serum CH50 of CH dogs fluctuated extensively and the mean serum CH50 of CH dogs during the neutropenic cycle greatly exceeded that of normal Collie dogs over the same test period. A close temporal relationship between the stage of the CH neutropenic cycle and the serum CH50 was observed. The mean serum CH50 during neutropenia was not significantly different from that observed when neutrophil counts were within normal range, both values being significantly higher than that of normal dogs. However, the mean serum CH50 during the period of neutrophil rebound was significantly (P less than 0.01) higher than that during the period of neutropenia or normal neutrophil counts. These data suggest that alterations in the production of complement components or regulatory proteins occur at regular intervals in CH dogs.