A family of epidermoid lung cancer models

A step section method for full histopathological assessment of carcinogen-affected target tissue during respiratory carcinogenesis

Studies of carcinogenesis that are not limited to overt neoplasms but also involve evaluations of preneoplastic stages require histopathological assessment of the entire carcinogen-affected tissue so that the true nature and sequence of the progressive process can be determined. The customary serial sectioning approach achieves this goal, but at an inordinate logistic cost. In studies of hamster bronchial carcinogenesis, a step section method was compared to a quasi-random approach and to the customary serial section method. The step section method achieved the same diagnostic completeness as serial sectioning, but at a two orders of magnitude reduction in costs.

Regression of bronchial epithelial cancer in hamsters

For bronchogenic carcinoma, if and when the sequential process of carcinogenesis is reversible is fundamental to chemoprevention research. In our hamster model, focally originating non-small cell lung carcinoma (NSCLC) develops via a reproducible sequential process of carcinogenesis by 180 days after endobronchial sustained-release implants (SRIs) of 10% benzo(a)pyrene. In this study, 114 hamsters received removable 10% benzo(a)pyrene SRIs. Short-term controls were sacrificed in 3 groups at 50, 65, and 80 days after SRI placement. Three experimental groups had SRIs removed at 50, 65, and 80 days after placement, and sacrifice was delayed until 100 to 180 days later. Long-term controls retained SRIs until sacrifice at 180 or 240 days after SRI placement. All long-term controls had NSCLC. Preneoplastic change was more common in 50- and 65-day controls, as compared with hamsters with equal duration of SRI exposure whose sacrifice was delayed until 100 to 180 days after SRI removal (p < 0.05). The 56% incidence of early NSCLC in hamsters sacrificed after 80 days of SRI exposure decreased to 5% in hamsters that had delayed sacrifice after SRI removal after 80 days of exposure. At the 10% benzo(a)pyrene dose used, hamster bronchial epithelium requires more than 80 days of continuous exposure to become irreversibly committed to NSCLC uniformly. Microinvasive NSCLC in hamsters often regresses, and it is not necessarily a precursor of overt invasive cancer. The removable SRI model provides new opportunities to evaluate chemoprevention of NSCLC and the related molecular-genetic control mechanisms.

Hamster bronchial carcinogenesis induced by carcinogen-containing sustained release implants placed endobronchially: a clinically relevant model

For several widely appreciated reasons, the Syrian hamster has been the most frequently used experimental animal in investigations of conducting airway carcinogenesis. To develop a model where bronchogenic cancer arises focally at a predetermined site, we used the Laskin-Kuschner self-retaining intrabronchial pellet principle, employing a carcinogen-containing silastic polymer sustained release implant (SRI). The SRI is placed in the right lower lobe bronchus via tracheostomy; when modified, the SRI can be removed without loss of the animal. Special preparation of the SRI implant site after fixation but prior to paraffin embedding allows for full histopathological examination of the carcinogen-affected target tissue. Logistic regression analysis of histological findings provides valid quantitative inter-regimen comparisons of histomorphic classifications suitable for determining modulation of carcinogenesis by external influences. Using this model, we demonstrated that the sequential progression of carcinogenesis (SPC) in hamster bronchus is similar to that which occurs in humans and in dogs, including both the ploidy increases that are progressive during the SPC, and the histological patterns of the induced cancers. We have shown genetic variation in susceptibility to carcinogenesis among inbred hamster strains, and have assessed effects of Bacillus Calmette-Guerin (BCG) immunostimulation on the SPC. Time/dose response studies were performed, as were comparisons between four polycyclic aromatic hydrocarbon carcinogens. Systemic administration of 5-azacytidine (AZC) soon after SRI placement was found to inhibit the SPC, to alter the ploidy changes during the SPC and in the eventual cancers, and to affect the degree of differentiation of the cancers. Studies using removable SRIs have assessed the duration of carcinogen exposure required to induce a neoplastic transformation that proceeds to cancer without further carcinogen exposure. Serial syngeneic transplantation of cancers arising in inbred animals has shown that the degree of tumor differentiation is affected by the extent of host immunocompetence, and has also led to development of models for study of the processes of metastasis.

Development of chemopreventive agents for lung and upper aerodigestive tract cancers

The lung and upper aerodigestive tract (oral cavity, larynx, pharynx, upper esophagus) will harbor the greatest proportion (approximately 20%) of estimated new cancer cases in 1992. The estimated mortality rate is even higher (32%), which is reflected in a 5-year survival rate of only 7% and 12% for esophageal and lung cancer, respectively. Tobacco use appears to remain the major cause of aerodigestive cancers despite efforts at primary prevention--cessation of exposure. Another strategy to decrease this public health problem is secondary prevention or chemoprevention. Cancer chemoprevention is defined as intervention with chemical agents before invasion to halt or slow the carcinogenic process; potential agents may include minor dietary constituents and pharmaceuticals. The main objective of the Division of Cancer Prevention and Control (DCPC), National Cancer Institute, is to develop promising chemopreventive drugs for use in humans. The testing of cancer chemopreventives for efficacy in the clinic differs from that of cancer treatment drugs. Chemopreventive drug trials involve healthy target populations, and the endpoints are reduced cancer incidence or mortality, or increased latency, with no to minimal toxicity. The lung and upper aerodigestive tract represent a unique opportunity for intervention in this setting. Even with cessation of tobacco exposure, the risk of cancer in the entire epithelium remains high for years due to the "field cancerization" effect. Some of the first chemopreventive trials made use of this system due to the availability of a study population with a tissue at demonstrably high risk for malignant progression. Much of the evidence for chemopreventive efficacy is in the oral cavity because of the well-defined epithelial neoplastic progression, the existence of well-established preclinical models, and relative ease of tissue monitoring and sampling. In one of the first randomized trials, Hong and co-workers demonstrated that 13-cis-retinoic acid prevents the appearance of second primary tumors in patients previously treated for squamous cell carcinomas of the oral cavity and upper respiratory tract. Even using a high risk population, chemoprevention trials involve large sample sizes, lengthy duration and follow-up, and high cost. To circumvent these problems, the use of intermediate biomarkers as surrogate endpoints is being explored. Intermediate biomarkers are defined as biological alterations in tissue (histological, genetic, biochemical, proliferative, differentiation-related) occurring prior to cancer development. In the oral cavity, studies using modulation of a histological intermediate biomarker, dysplastic leukoplakia, as the endpoint have demonstrated response to a retinoid.(ABSTRACT TRUNCATED AT 400 WORDS)

Metastatic behaviour of canine lung carcinoma in autochthonous and xenotransplant hosts

Specimens from 24 chemically induced canine non-small cell lung cancers (NSCLC) were xenotransplanted into nude mice. Twenty-one tumour lines were established in serial transplantation; four were from NSCLC that arose from orthotopically induced NSCLC in four dogs, and 17 were from NSCLC that arose heterotopically in 15 subcutaneous bronchial autografts (SBA) in seven dogs. Distant metastases developed in recipients of two orthotopic NSCLC after three and eight consecutive tumour growth cycles; no metastases have occurred after three and six growth cycles of two other orthotopic tumour lines. Recipients of eight heterotopic tumour lines developed metastases after 3-9 consecutive tumour growth cycles, while no metastases have occurred after 4-11 growth cycles in recipients of nine other heterotopic tumour lines. In three instances, both metastasizing and non-metastasizing tumour lines resulted from NSCLC that arose in different SBAs in the same dog. These findings indicate that, in the canine SBA bronchogenic cancer model as expanded by tumour xenotransplantation, those molecular events involved in both the initiation and the full progression of a single cancer may be investigated serially and concomitantly.

Lung cancer model for study of the metastatic process

In our hamster model of focal, chemically induced nonsmall cell lung cancer (NSCLC), we studied metastases in autochthonous hamster hosts (n = 300) and in syngeneic hamster and nude mice recipients (n = 230) of serial tumor transplants. Metastases in autochthonous hosts and transplant recipients occurred in regional lymph nodes, liver, and adrenals. In autochthonous host hamsters no metastases were noted from microinvasive (n = 112) or visible cancer less than 3.0 mm in diameter (n = 66); the incidence of metastasis was 8.2% (4/49) from 3- to 10-mm cancers and 22% (16/73) from cancers 10 mm in diameter or larger (p less than 0.05). Serial transplants were used to evaluate the metastatic propensity of 20 primary and six metastatic NSCLCs. Six primary NSCLCs that metastasized in the autochthonous host and six metastatic NSCLCs all metastasized promptly in recipients. This expression of metastatic potential was significantly different (p less than 0.05) from 14 primary cancers without autochthonous host metastases. Eight of the 14 caused no metastases in recipients, even after 5 to 11 tumor growth cycles; metastases occurred from the other six primary NSCLC after 3 to 12 tumor growth cycles in transplant recipients. Primary hamster NSCLCs metastasize in the autochthonous host with a frequency and a distribution pattern similar to human NSCLCs. A new model to study serially the cellular changes that govern the process of metastasis in NSCLC has been developed.

Effects of 5-azacytidine in Syrian golden hamsters: toxicity, tumorigenicity, and differential modulation of bronchial carcinogenesis

5-Azacytidine (AZC) was studied in a lung cancer model in outbred and syngeneic (F1D) hamsters wherein benzol[a]pyrene (BP) from sustained release implants (SRI) induces preneoplastic mucosal changes which progress to bronchogenic cancer. In pilot studies to evaluate AZC toxicity, a dose schedule of 5 mg/kg biweekly was found suitable and was then used for long-term administration in all subsequent studies. Three groups of outbred hamsters were studied: BP SRi alone (n = 60), BP SRI + AZC (n = 60), and AZC alone (n = 54). AZC treatment was begun 3-5 days after SRI placement. Sixty-one days after the start of the experiment, seven or eight hamsters were sacrificed from each group. Later sacrifices were at 3-week intervals in groups receiving BP SRI and at 6-week intervals in the AZC only group. Four groups of F1D syngeneic hamsters were studied: BP SRI alone (n = 50); BP + AZC starting 3-5 days after SRI placement and continuing until death (n = 52); BP + AZC from 3 to 5 days until 75 days after SRI placement (n = 49); BP + AZC starting 80 days after SRI placement and continuing until death (n = 52). Hamsters (n = 9-14) from each group were sacrificed at 120, 150, 180, and 220 days after SRI implantation. AZC alone was not carcinogenic under these conditions. Both outbred and F1D hamsters treated with early or continuous AZC had slower rates of neoplastic change from BP SRI than did animals receiving BP SRIs alone or BP + late AZC. The incidence of epidermoid cancer were the same for all regimens, but the tumors in those receiving AZC early in carcinogenesis were smaller than in those receiving late or no AZC. The incidences of nonepidermoid cancer were lower in those receiving AZC during early carcinogenesis, and larger tumors were noted in the absence of AZC. Thus, within the study period in this unique hamster lung cancer model, AZC given early in carcinogenesis inhibited only the later (promotional) phase of BP epidermoid carcinogenesis, but inhibited all phases of nonsquamous cancer development induced by BP. This differential modulation of bronchial carcinogenesis, which occurs from AZC given during preneoplastic stages, may prove useful for delineating molecular mechanisms underlying specific phenotypic types of bronchogenic cancers.