The Biology Behind Protease Inhibitors in Oral Carcinogenesis and Chemoprevention

We are honored to write the commentary for the inaugural report of “The Biology Behind. . . ” series in Clinical Cancer Research. We confess at the outset, however, that the reader conducting the most thorough survey of the cancer literature would not turn up any publications on BBI with either of our names in the author list (before now). Nevertheless, the scientific roots of “Clinical Modulation of Oral Leukoplakia and Protease Activity by Bowman-Birk Inhibitor Concentrate in a Phase IIa Chemoprevention Trial,” by Armstrong et al. (1), grow within two respective fields of our specialized research: translational study of oral carcinogenesis and chemoprevention (Lippman) and the contribution of proteolysis to tumor progression (Matrisian). The three major avenues of biology behind BBI study in oral carcinogenesis are (a) the long-term development of OPLs as a clinical and translational model for assessing drug activity; (b) preclinical (and early clinical) study of BBI itself; and (c) broader basic study of PIs. Our charge is to “translate” the contribution of the background biology to the currently reported findings on clinical BBIC activity. This background biology and clinical BBIC study together raise important issues concerning the larger picture of drug development for cancer chemoprevention, which we also discuss below. The study of PIs in preclinical cancer chemoprevention began about 30 years ago (2). The preclinical study of BBI began about 15 years ago in colon and oral-cavity carcinogenesis and now involves many sites that could be studied clinically (3). The authors’ choice of OPLs for the early clinical study of BBIC, however, is commendable. Work in the OPL model has been a driving force behind the entire field of chemoprevention, which matured recently with the hard-earned FDA approvals of tamoxifen (for reducing breast cancer risk) and celecoxib (for controlling familial adenomatous polyposis; Ref. 4). The OPL model offers several important research advantages, including lesions that are easily monitored and sampled; an association with clonal expansion and the risk of second primary tumors; and statistical models for analyzing biomarker modulations and correlations. These methodological issues have been worked out in the more than 20 years of clinical and translational study of retinoids in OPLs (phenotype-genotype reversal/suppression of OPLs and extension to the prevention of second primary tumors associated with head-and-neck cancer), which have been documented extensively elsewhere (5–7). The current clinical/translational study of BBIC is approximately where the study of retinoids was 20 years ago but likely will benefit from the earlier work. OPL study has produced major translational findings involving retinoic acid receptor-b loss and retinoid regulation (8), clonal expansion indicated by 3p14 and 9p21 loss of heterozygosity (9), p53, and genetic instability (10), among other biomarkers associated with carcinogenic progression and/or pharmacological retinoid activity (Ref. 4; Fig. 1), and the first systematic statistical model for analyzing biomarkers within chemoprevention trials (11). The OPL model also offers a specific advantage for BBIC study, which is the ease of topical delivery. In the current Phase IIa study, a solution of BBIC was held in the mouth (before swallowing; Ref. 1). Protease expression and activation patterns beginning to be worked out in head and neck cancer will be extended to OPLs [adding to the recent findings of increased protease activity in OPLs and “normal” oral mucosa of smokers (Refs. 1 and 12; see Fig. 1)]. Because BBIC has preclinical activity in several sites besides the oral cavity, the early clinical development of BBIC within the welldeveloped OPL model may be relevant to clinical BBIC study elsewhere. The BBI family consists of many forms and isoforms of natural polypeptide serine PIs and is found in the seeds of legumes (e.g., Soybean, chickpea, and peanut) and other plants (e.g., barley). Plant serine PIs function as natural insecticides. BBIs are one of the two main families of serine plant PIs (the other is the Kunitz-type PIs). BBIs come from dicotyledonous or monocotyledonous seeds and beans, and the amino acid sequences of about 100 BBIs have been worked out, as have the three-dimensional structures of a few (13). The classic and first BBI, the one that is most commonly referred to in the literature, is a protein that was identified in soybeans by Bowman in the 1940s and purified by Birk in the early 1960s (14). This molecule is a typical dicotyledonous BBI and has a Mr of 8,000, 71 amino acids, and two separate PI sites: subdomain 1 (NH2terminal) and subdomain 2 (COOH-terminal) for trypsinand chymotrypsin-like serine proteases, respectively. The more important PI site for chemoprevention is subdomain 2, suggesting the importance of chymotrypsin inhibition for BBI chemoprevention activity (Ref. 3; recent findings on subdomain specificity for protease binding are discussed later). The agent in the study reported in this issue of Clinical Cancer Research (p. 4684) by Armstrong et al. was not pure BBI, but BBIC, a crude acetone-defatted soybean-flour extract containing the classic BBI, four other soy PIs (with trypsin, but 1 Supported in part by Grants CA16672 and CA68485 from the National Cancer Institute, NIH, Department of Health and Human Services. S. M. Lippman holds the Margaret and Ben Love Professorship in Clinical Cancer Care. L. M. Matrisian is an Ingram Professor of Cancer Research. 2 To whom requests for reprints should be addressed, at Department of Clinical Cancer Prevention, Box 236, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030-4009. Phone: (713) 745-3672; Fax: (713) 794-4679; E-mail: [email protected]. 3 The abbreviations used are: BBI, Bowman-Birk inhibitor; BBIC, Bowman-Birk inhibitor concentrate; OPL, oral premalignant lesion; PI, protease inhibitor; FDA, United States Food and Drug Administration; COX, cyclooxygenase; MMP, matrix metalloproteinase; MMPI, matrix metalloproteinase inhibitor. 4599 Vol. 6, 4599–4603, December 2000 Clinical Cancer Research