Studies on the red marrow dosimetry in radioimmunotherapy: An experimentalinvestigation of factors influencing the radiation-induced myelotoxicity in therapy with beta-, Auger/conversion electron-, or alpha-emitters

Usually, the red marrow (RM) is the first dose-limiting organ in radioimmun otherapy. However, several studies have obtained only poor correlations bet ween the marrow doses and the resulting toxicities. Furthermore, RM doses a re mostly not determined directly but are derived from blood doses by assum ing a ratio that is, over time for the respective conjugates, more or less constant between blood and marrow activities. The aim of this study was to determine, in a mouse model, this RM:blood activity ratio for various immun oconjugates, to investigate whether there may be differences between comple te IgG and its fragments with various labels (I-125/131 versus In-111, Y-88 /90, or Bi-213), and to analyze, in more detail, factors other than just to tal dose, such as dose rate or relative biological effectiveness factors, t hat may influence the resulting myelotoxicity. The maximum tolerated activities (MTAs) and doses (MTDs) of several murine, chimeric, and humanized immunoconjugates as complete IgG or fragments (F(a b)(2) and Fab), labeled with beta(-)-emitters (such as I-131 or Y-90), Auge r electron-emitters (such as I-125 or In-111), or alpha-emitters (such as B i-213) were determined in nude mice. Blood counts were monitored at weekly intervals; bone marrow transplantation was performed to support the assumpt ion of the RM as dose-limiting. The radiation dosimetry was derived from bi odistribution data of the various conjugates, accounting for cross-organ ra diation; besides the major organs, the activities in the blood and bone mar row (and bone) were determined over time. Whereas no significant differences were found for the RM:blood ratios betwe en various IgG subtypes, different radiolabels or various time points, diff erences were found between IgG and bi- or monovalent fragments: typically, the RM:blood ratios were approximately 0.4 for IgG, 0.8 for F(ab')(2), and 1.0 for Fab'. Nevertheless, at the respective MTAs, the RM doses differed s ignificantly between the three conjugates: e.g., with I-131-labeled conjuga tes, the maximum tolerated activities were 260 mu Ci for IgG, 1200 mu Ci fo r F(ab)(2), and 3 mCi for Fab, corresponding to blood doses of 17, 9, and 4 Gy, respectively. However, initial dose rates were 10 times higher with Fa b as compared to IgG, and still 3 times higher as compared to F(ab),; inter estingly, all three deliver similar to 4 Gy within the first 24 h. The MTDs of all three conjugates were increased by BMT by approximately 30%. Simila r observations were made for Y-90-conjugates. Higher RM doses were tolerate d with Anger-emitters than with conventional beta(-)-emitters, whereas the MTDs were similar between alpha- and beta(-)-emitters. In accordance to dos e rates never exceeding those occurring at the single injection MTA, two su bsequent injections of two doses of 80% of the single shot MTA of I-131- or Y-90-labeled Fab' and two doses of 100% of the single shot MTA of Bi-213-l abeled Fab' were tolerated without increased lethality, if administered 24- 48 h apart. In contrast, reinjection of bivalent conjugates was not possibl e within 6 weeks. These data suggest that the RM:blood activity ratios differ between IgG and fragments, although there is no anatomical or physiological explanation fo r this phenomenon at this point. In contrast to the current opinion, indica tion for a strong influence of the dose rate (or dose per unit time), not o nly total dose, on the resulting toxicity is provided, whereas the influenc e of high-linear energy transfer (alpha and Auger/conversion electrons) ver sus low-linear energy transfer (beta and gamma) type radiation seems to be much lower than expected from previous in vitro data. The lower myelotoxici ty of Auger-emitters is probably due to the short path length of their low- energy electrons, which cannot reach the nuclear DNA if the antibody is not internalized into the stem cells of the RM.