Imaging with bispecific antimyosin Fab-anti-DTPA Fab and Tc-99m polymers to demonstrate reduction of doxorubicin cardiotoxicity using polymer-drug conjugates

Objectives The limitation of doxorubicin (Dox) for optimal chemotherapy is its cardiotoxicity. Covalent conjugation of Dox to polymers have reduced its cardiotoxicity. A bispecific anti-myosin Fab-anti-DTPA Fab (bsAM-AD) was used to pretarget and cardiotoxicity was imaged by targeting Tc-99m-labeled DTPA-succinyl polylysine polymers (DSPL).

Methods Balb/c mice were injected i.v. with 10 mg/kg Dox or equivalent DTPA Dox-polyglutamic acid polymers (D-Dox-PGA) biweekly for 3 doses. Mice injected with Dox died by day 20. Therefore, 4 mice from this group were pretargeted with bsAM-AD (10 μg) on day 6 and imaged with Tc-99m DSPL(37 MBq) on day 7. D-Dox PGA and control mice were imaged on day 27 and 38. Total body weight (TBW) was measured daily for all surviving mice.

Results About 15 % loss of TBW occurred in Dox treated mice in 18 days (84.17 % of starting TBW) and all died by 20 days. D-Dox-PGA group showed no loss of total body weight for the length of the experiment (100.3% at 18 days and 100.4% at 38 days). Untreated control TBW at 18 and 38 days were 114.4% and 106.8% respectively). bsAM-AD /Tc-99m DSPL images showed unequivocal cardiac activity in the Dox group (1.90+0.25 %ID/g). D-Dox-PGA mice (0.40+/-0.04 %) had lower cardiac activity than the Dox group(p<0.0001), but was greater than that of control mice (0.20+/-0.04, p<0.001).

Conclusions D-Dox-PGA has been demonstrated to target HER2 receptor positive human mammary tumor xenografts after pretargeting with HER2 specific bsAb complexes. This study reports the reduction of cardiotoxicity with D-Dox-PGA relative to Dox administration by pretargeted imaging with bsAM-AD and Tc-99m-DSPL. Cummulative equivalent dose of 4 times the MTD (7.5 mg/kg) of Dox, was well tolerated by the experimental mice