Rhona Stein

// David M. Goldenberg 1, 2 , Rhona Stein 1 and Robert M. Sharkey 1, 3 1 Center for Molecular Medicine and Immunology, Belleville, NJ, USA 2 IBC Pharmaceuticals, Inc., Morris Plains, NJ, USA 3 Immunomedics, Inc., Morris Plains, NJ, USA Correspondence to: David M. Goldenberg, email: dmg.gscancer@att.net Keywords: TROP-2; TACSTD2; sacituzumab govitecan; antibody-drug conjugates; immunotherapy Received: April 30, 2018      Accepted: May 31, 2018      Published: June 22, 2018 ABSTRACT TROP-2 is a glycoprotein first described as a surface marker of trophoblast cells, but subsequently shown to be increased in many solid cancers, with lower expression in certain normal tissues. It regulates cancer growth, invasion and spread by several signaling pathways, and has a role in stem cell biology and other diseases. This review summarizes TROP-2’s properties, especially in cancer, and particularly its role as a target for antibody-drug conjugates (ADC) or immunotherapy. When the irinotecan metabolite, SN-38, is conjugated to a humanized anti-TROP-2 antibody (sacituzumab govitecan), it shows potent broad anticancer activity in human cancer xenografts and in patients with advanced triple-negative breast, non-small cell and small-cell lung, as well as urothelial cancers.

CD74 is an integral membrane protein that functions as a MHC class II chaperone. Moreover, it has recently been shown to have a role as an accessory-signaling molecule and has been implicated in malignant B-cell proliferation and survival. These biological functions combined with expression of CD74 on malignant B cells and limited expression on normal tissues implicate CD74 as a potential therapeutic target. The anti-CD74 monoclonal antibody LL1 has been humanized (hLL1 milatuzumab or IMMU-115) and can provide the basis for novel therapeutic approaches to B-cell malignancies, particularly because this antibody shows rapid internalization into CD74+ malignant cells. This article reviews the preclinical evaluations of LL1, its humanized form, and isotope, drug, and toxin conjugates. These studies show that unconjugated hLL1 and conjugates of hLL1 constructs with radioisotopes, doxorubicin, and frog RNase have high antitumor activity in non-Hodgkin's lymphoma and multiple myeloma in vitro and in tumor xenograft models. Single-dose studies of hLL1 in monkeys showed no adverse effects but did decrease circulating B and T lymphocytes and natural killer cells. When evaluated in combination with rituximab, either equivalent or improved efficacy, compared with either antibody alone, was observed. CD74 is a new candidate target for the immunotherapy of neoplasms expressing this antigen, which can be exploited using either a naked antibody or conjugated to isotopes, drugs, or toxins.

Cyclooxygenase-2 (COX-2) is an important cellular target for both therapy and/or prevention of inflammatory disorders and cancer. The advent of selective COX-2 inhibitors now allows a more precise and safer treatment approach. The screening of an array of cancer cell lines for growth inhibitory effects of COX-2-selective and -nonselective inhibitors, including celecoxib (Celebrex) and rofecoxib (Vioxx), produced two unanticipated findings. Firstly, the antiproliferative effects of celecoxib were noted to be of very similar magnitude for both hematopoietic and epithelial cancer cell lines. Most hematopoietic cell lines had no detectable COX-2 expression by reverse transcription-PCR, and none expressed COX-2 protein. In addition, COX-2-negative epithelial lines were found to have IC50s for celecoxib that were very similar to their COX-2+ counterparts. Thus, important antiproliferative effects were observed that were independent of both the cell lineage and COX-2 status. Secondly, it was also observed that COX-2 inhibitor drugs, celecoxib and rofecoxib, with similar COX-2-selectivity and clinical efficacy for inflammatory indications, differed significantly in their in vitro antiproliferative effects on cancer cell lines. IC50s of 35-65 microM were observed for celecoxib across this entire panel of cell lines. Finally, no difference in the mode or degree of cytotoxicity was apparent between cell lines, because similar levels of apoptosis were observed in COX-2+ and -negative cell lines after treatment with celecoxib, with correspondingly lower levels after rofecoxib treatment. These data are important in that they provide the first direct comparison of epithelial and hematopoietic cancer cell lines, as well as a direct comparison of the in vitro anticancer effects of the two clinically available COX-2 inhibitors.

Sixteen patients with non-Hodgkin's lymphoma were infused with 6.2 to 58.2 mCi (0.2 to 3.9 mg) doses of radioactive iodine (131I)-labeled LL2 immunoglobulin G (IgG) or F(ab')2, in order to study antibody distribution, pharmacokinetics, dosimetry, toxicity, tumor targeting, and therapy. LL2 is a murine IgG2a monoclonal antibody (MAb) reactive with B cells and non-Hodgkin's B-cell lymphoma. In a series of five assessable therapy patients, doses as small as 30 mCi 131I-LL2 IgG or F(ab')2 resulted in tumor responses (two partial remissions, two mixed and minor responses, and one no response), while one patient receiving diagnostic doses as low as 6.2 mCi showed a partial remission for 1 year and a complete remission after a second low radiation dose. No acute toxicities were noted, and only myelotoxicity accompanied therapeutic doses, with grade IV marrow toxicity seen in three of seven patients receiving total doses of about 50 mCi. Dosimetry calculations showed spleen and tumor dose rules of about 4.6 cGy/mCi, which was three to four times the dose to other organs. Despite the administration of relatively low doses of LL2 (0.2 to 3.9 mg), 82% of 60 known extrasplenic lymphoma sites were imaged. Serum clearance showed an average distribution half-life (T1/2) of 2.1 hours and an elimination T1/2 of 32.0 hours. The average total-body clearance T1/2 was 43 to 45 hours. LL2's antigenic target does not appear to be shed in high amounts into the circulation. Three of eight patients having at least two injections showed a human antimouse antibody response. These patients may have been presensitized to animal protein. An interesting observation in this study was the marked drop in circulating B lymphocytes after the administration of radioiodinated LL2 or anticarcinoembryonic antigen MAbs, suggesting that this is a nonspecific radiation effect and not necessarily related to the binding of MAb to normal B cells.