Hyper-radiosensitivity affects low-dose acute myeloid leukemia incidence in a mathematical model

In vitro experiments show that the cells possibly responsible for radiation-induced acute myeloid leukemia (rAML) exhibit low-dose hyper-radiosensitivity (HRS). In these cells, HRS is responsible for excess cell killing at low doses. Besides the endpoint of cell killing, HRS has also been shown to stimulate the low-dose formation of chromosomal aberrations such as deletions. Although HRS has been investigated extensively, little is known about the possible effect of HRS on low-dose cancer risk. In CBA mice, rAML can largely be explained in terms of a radiation-induced Sfpi1 deletion and a point mutation in the remaining Sfpi1 gene copy. The aim of this paper is to present and quantify possible mechanisms through which HRS may influence low-dose rAML incidence in CBA mice. To accomplish this, a mechanistic rAML CBA mouse model was developed to study HRS-dependent AML onset after low-dose photon irradiation. The rAML incidence was computed under the assumptions that target cells: (1) do not exhibit HRS; (2) HRS only stimulates cell killing; or (3) HRS stimulates cell killing and the formation of the Sfpi1 deletion. In absence of HRS (control), the rAML dose-response curve can be approximated with a linear-quadratic function of the absorbed dose. Compared to the control, the assumption that HRS stimulates cell killing lowered the rAML incidence, whereas increased incidence was observed at low doses if HRS additionally stimulates the induction of the Sfpi1 deletion. In conclusion, cellular HRS affects the number of surviving pre-leukemic cells with an Sfpi1 deletion which, depending on the HRS assumption, directly translates to a lower/higher probability of developing rAML. Low-dose HRS may affect cancer risk in general by altering the probability that certain mutations occur/persist.

     

Degradation of radioiodinated B cell monoclonal antibodies: inhibition via a FCγ -receptor-II-mediated mechanism and by drugs

 Our aim is to treat patients with B cell malignancies with radioimmunotherapy using monoclonal antibodies (mAb) such as CD19, CD20 and CD22. In this study we investigated the rate of internalization and catabolism of these mAb. After 24 h at 37°C, 20% – 25% of initially cell-bound ¹²⁵I-CD19 mAb and ¹²⁵I-CD22 mAb was degraded in B cells, whereas almost no degradation occurred after binding of ¹²⁵I-CD20 mAb. For B cells expressing Fcγ receptor II (FcγRII), isotype-dependent degradation was noted as the CD19 IgG1 mAb showed an enhanced degradation rate compared to the switch variant IgG2a. The effect of various pharmaceutical agents that delay the internalization or subsequent degradation of mAb was evaluated. The degradation was inhibited most effectively by a combination of etoposide and vinblastine, resulting in accumulation of radioactivity in the target cell. Also the simultaneous application of CD20 or CD22 with ¹²⁵I-CD19 mAb or of CD20 with ¹²⁵I-CD22 mAb proved to be a potent inhibitor of the rapid degradation of these mAb, by inhibiting internalization via an FcγRII-mediated mechanism. Both methods of reducing the degradation of radioiodinated mAb are expected to prolong irradiation of malignant B cells and consequently result in an enhanced therapeutic effect in vivo. Received: 22 September 1995 / Accepted: 13 November 1995