SYNOPSIS Platelets from six normal subjects were labelled with radiochromate and reinfused into their donors. Subsequently, the effect of intravenously injected heparin upon the levels of circulating platelets and platelet radioactivity was studied. Although reductions in platelet counts occurred in three subjects following single injections of 10,000 units, these changes were not reflected in the levels of circulating platelet radioactivity. It is concluded that the alterations in platelet counts observed were probably due to an effect of heparin in vitro. The experimental evidence that heparin induces thrombocytopenia in vivo, in man, is unconvincing.

The DNA vaccine, AV-1959D, targeting N-terminal epitope of Aβ peptide, has been proven immunogenic in mice, rabbits, and non-human primates, while its therapeutic efficacy has been shown in mouse models of Alzheimer's disease (AD). Here we report for the first time on IND-enabling biodistribution and safety/toxicology studies of cGMP-grade AV-1959D vaccine in the Tg2576 mouse model of AD. We also tested acute neuropathology safety profiles of AV-1959D in another AD disease model, Tg-SwDI mice with established vascular and parenchymal Aβ pathology in a pre-clinical translational study. Biodistribution studies two days after the injection demonstrated high copy numbers of AV-1959D plasmid after single immunization of Tg2576 mice at the injection sites but not in the tissues of distant organs. Plasmids persisted at the injection sites of some mice 60 days after vaccination. In Tg2576 mice with established amyloid pathology, we did not observe short- or long-term toxicities after multiple immunizations with three doses of AV-1959D. Assessment of the repeated dose acute safety of AV-1959D in cerebral amyloid angiopathy (CAA) prone Tg-SwDI mice did not reveal any immunotherapy-induced vasogenic edema detected by magnetic resonance imaging (MRI) or increased microhemorrhages. Multiple immunizations of Tg-SwDI mice with AV-1959D did not induce T and B cell infiltration, glial activation, vascular deposition of Aβ, or neuronal degeneration (necrosis and apoptosis) greater than that in the control group determined by immunohistochemistry of brain tissues. Taken together, the safety data from two different mouse models of AD substantiate a favorable safety profile of the cGMP grade AV-1959D vaccine supporting its progression to first-in-human clinical trials.

SUMMARY.A technique for the labelling of human blood platelets with radioactive sodium chromate is described, and is compared with those previously in use. The uptake of radiochroinate by platelets is greatest when the platelet suspension is of small volume, plasma protein is absent, and the sodium chromate concentration is 0-2 to 3-0 ng. per lO'" platelets. The rate of the reaction is not a£fccted by its duration up to 40 minutes, but is enhanced by an increase in temperature. About one-third of platelets in suspensions labelled as described are viable, and survive in the circulation for 8-10 days.

When freshly collected human platelets are transfused into normal recipients, an appreciable proportion disappears extremely rapidly from the circulation and does not reappear. This proportion increases with the number of manipulations performed in the preparation of the platelet concentrate (Gardner, Howell and Hirsch, 1954), and with the period of storage before infusion (Baldini, Costea and Dameshek, 1960). Those platelets which remain in the circulation after 24 hours—so‐called ‘viable’platelets—disappear slowly from the blood in the course of a further 7–10 days (see reviews by Najean and Bernard, 1960, and Odell and Kniseley, 1962). This period of platelet survival in the circulation is similar to that determined by labelling platelets in vivo with di‐isopropylfluorophosphonate‐32 P (DF32 P) (Leeksma and Cohen, 1956; Zucker, Ley and Mayer, 1961). Following the infusion of labelled platelets into normal recipients, it has also been shown that the platelet‐bound radioactivity in the blood increases after 30 minutes to reach a maximum within 24–48 hours of infusion (Adelson, Rheingold and Crosby, 1957; Reisner, Keating, Friesen and Loeffler, 1958; Aas and Gardner, 1958; and others). This is contrary to what might be anticipated. A delayed increase in the number of circulating platelets has also been shown to occur when unlabelled platelets are given to thrombocytopenic subjects (Aas and Gardner, 1958). No such change is observed, however, when DF32P is injected to label circulating platelets (Zucker et al., 1961); this suggests that the phenomenon is due to changes which occur during the removal of platelets from the body and/or their manipulation in vitro. The fate of those infused platelets which are immediately and irreversibly cleared from the circulation is unknown; nor is it known where the potentially viable platelets reside before their return to the circulation, nor where they are finally destroyed. Surface‐counting procedures using platelets labelled in vitro with 51Cr have led some workers to suggest that temporary accumulation occurs in the liver, the lungs and possibly the spleen, and that the liver and spleen play some role in their final destruction (Aas and Gardner, 1958; Najean and Bernard, 1960; Najean, Larrieu and Bernard, 1961). This study is concerned particularly with the sequence of events in the brief period immediately following the infusion of 51Cr‐labelled human platelets—a period in which some of them disappear rapidly from the circulation, and later return to it. We have carried out 36 studies of these events.

With the use of radiochromium as a label, reduced platelet survivals were demonstrated in eight of 12 patients with polycyth