Increased sensitivity of antithymocyte globulin-treated newborn rat assay for metastasis

Tumorigenic and metastatic abilities of FL, transformed human amnion, Vero and LLC-MK2, continuous monkey kidney cell lines (CCLs), none of which produced lung metastases in nude mice, were studied in ATG-treated newborn Wistar rats. Effects of subcutaneous (s.c.) and intraperitoneal (i.p.) inoculation of 10(6) and 10(7) cells were compared. l.p. inoculation of the same number of FL cells produced larger primary tumors, increased incidence of lung metastasis and more metastases per rat than s.c. inoculation. The same was true of the higher inocula administered by either route as compared to the smaller inocula. The CCLs (Vero at passages 320 and 323, LLC-MK2 at passage 306) inoculated either s.c. or i.p. induced small tumors regressing in all animals. After s.c. inoculation of 10(6) Vero or LLC-MK2 cells lung metastases did not develop in any rats, but 10(7) inoculum and i.p. administration of both size inocula caused single lung metastases. Thus, increased sensitivity of the rat system was achieved by larger size of the s.c. inoculum or the i.p. route of inoculation. Highly sensitive assay is especially important for evaluation of possible metastatic abilities of CCLs.