Apoptotic death of human promyelocytic leukemia HL-60 cells in vivo after flavopiridol therapy. (a) Histological section of a s.c. HL-60 tumor (100×) removed from a nontreated control nude mouse, showing a large number of predominantly viable and dividing malignant blasts. (b) ApopTag immunostaining of the same HL-60 tumor as (a), showing the sporadic presence of brown-stained cells indicative of rare spontaneous apoptosis. (c) Histological section of an HL-60 tumor (100×) 96 hours after initiation of treatment with daily IV bolus injection of flavopiridol, showing multiple fragments of condensed chromatin (“apoptotic bodies”) indicative of cell death through apoptosis. (d) ApopTag immunostaining of an HL-60 tumor 96 hours after initiation of treatment with bolus IV flavopiridol. Multiple cells densely stained brown indicate DNA fragmentation. (e) Agarose gel analysis of DNA isolated from SUDHL-4 nontreated lymphoma (lane 2), and DNA isolated from a tumor 72 hours after initiation of treatment with flavopiridol (lane 3), which shows the typical “ladder” pattern of internucleosomal cleavage of DNA into multiples of 180 to 250 bp. Lane 1 includes DNA molecular mass markers.