Relapse is the leading cause of mortality in children with acute lymphoblastic leukemia (ALL). Among chemotherapeutics, thiopurines are key drugs in the backbone of ALL combination therapy. Using whole-exome sequencing, we identified relapse-specific mutations in phosphoribosyl pyrophosphate synthetase 1 (PRPS1), a rate-limiting purine biosynthesis enzyme, in 24/358 (6.7%) relapse B-ALL cases. All individuals who harbored PRPS1 mutations relapsed early on-treatment, and mutated ALL clones expanded exponentially prior to clinical relapse. Our functional analyses of PRPS1 mutants uncovered a new chemotherapy resistance mechanism involving reduced feedback inhibition of de novo purine biosynthesis and competitive inhibition of thiopurine activation. Notably, the de novo purine synthesis inhibitor lometrexol can effectively abrogate PRPS1 mutant-driven drug resistance. Overall these results highlight the importance of constitutive activation of de novo purine pathway in thiopurine resistance, and offer therapeutic strategies for the treatment of relapsed and resistant ALL.

The in vivo(212)Pb/(212)Bi generator is promising for application in targeted alpha therapy (TAT) of cancer. One main limitation of its therapeutic application is due to potential release of (212)Bi from the radioconjugate upon radioactive decay of the mother nuclide (212)Pb, potentially leading to irradiation of healthy tissue. The objective of the present work is to assess whether the chelate CHX-A''-DTPA (N-(2-aminoethyl)-trans-1,2-diaminocyclohexane-N,N',N''-pentaacetic acid) bound to a biological carrier molecule may be able to re-complex released (212)Bi under in vivo conditions to limit its translocation from the target site. CHX-A''-DTPA was bound to bovine gamma globulin (BGG) to mimic a model conjugate and the stability of the Bi-CHX-A''-DTPA-BGG conjugate was studied in blood serum by ultrafiltration. TRLFS experiments using Cm(III) as a fluorescent probe demonstrated that linking CHX-A''-DTPA to BGG does not affect the coordination properties of the ligand. Furthermore, comparable stability constants were observed between Bi(III) and free CHX-A''-DTPA, BGG-bound CHX-A''-DTPA and DTPA. The complexation constants determined between Bi(III) and the chelate molecules are sufficiently high to allow ultra trace amounts of the ligand to efficiently compete with serum transferrin controlling Bi(III) speciation in blood plasma conditions. Nevertheless, CHX-A''-DTPA is not able to complex Bi(III) generated in blood serum because of the strong competition between Bi(III) and Fe(II) for the ligand. In other words, CHX-A''-DTPA is not "selective" enough to limit Bi(iii) release in the body when applying the (212)Pb/(212)Bi in vivo generator.