Resistance to alkylating agents and tumour differentiation in xenografts of small cell lung cancer.

Small cell bronchial carcinoma (SCC) xenografts with differing sensitivity to cyclophosphamide (CY) were investigated using a variety of techniques. Two xenografts (HX78 and HX88) were relatively sensitive to CY, one xenograft (HX72) was inherently resistant to CY and a fourth xenograft (HX78Cy) was a CY induced resistant subline of HX78 and was unstable when maintained without CY exposure. Conventional light microscopy, cytology and electron microscopy examination of the xenografts revealed appearances consistent with SCC. An antikeratin antibody demonstrated the presence of keratin in the inherent and the induced resistant xenografts (and in the unstable revertant) but not in the two sensitive xenografts; the presence of keratin suggested squamous differentiation. Monolayer culture morphology of the sensitive HX78 and the unstable revertant was anchorage independent with the cells forming floating aggregates; the induced resistant subline of this xenograft (HX78Cy) showed, by contrast, flattened, angular adherent cells. Beta HCG production was detected in the monolayer culture supernatant of sensitive HX78 cells; the level of production of beta HCG was increased in the induced resistant HX78Cy cells. Karyotype and flow cytometry studies were also performed. The morphological responses of small cell lung cancer to treatment are discussed.