How selective are agar cultures for malignant transformation?

In vitro assays that permit cloning of tumour cells in soft agar have been improved during the last 5 years. Two of them (2, 10) are claimed to be useful as test systems for the screening of new anticancer drugs and even for drug sensitivity testing of individual human tumours in the devicing of individualized cancer chemotherapy regimens. Three assays were investigated for this report: those of Toshio Kuroki (TK) (11) and Hamburger and Salmon (HS) (5, 10) and that in use for bone marrow cell cultures (BM) (3). Cells of various origins were tested for their growth capacity and colony formation in these three assays. Included were cells of 10 established lines classified as malignant or nonmalignant according to the in vivo malignancy test (6). Cells freshly derived from two tumours ans those from five tumours after 2-10 passages in monolayer culture were also used as test cells. The BM assay gave the best results. Up to now, a 100 per cent correlation has been found between the in vivo and in vitro test. Investigations are under way to determine whether this assay can also be used as a transformation assay using cells with a low transformation rate.