PARP-1 and PARP-2 are nuclear enzymes activated by DNA strand breaks and are involved in recruitment of DNA repair proteins to sites of damage. PARP-1 and PARP-2 share partially overlapping functions, as shown by the embryonic lethality of double knockout mice compared to animals with single ablation either gene, which are viable. Despite this partial functional redundancy, PARP-1, but not PARP-2, inhibition has been shown to be synthetically lethal with defects in homologous recombination such as BRCA gene mutations.
All PARP inhibitors that are currently under clinical investigation inhibit both PARP-1 and PARP-2, due to the high sequence similarity between the catalytic domains of the two enzymes.
A third member of the family, PARP-3, has lower homology with PARP-1 and -2 and is much less well characterized, although it is known that its activity is stimulated by double strand breaks and that it promotes DNA repair through an error prone repair pathway, non-homologous end joining ( NHEJ ).
PARP-3 inhibition is thus presumed to be synthetically lethal with other, alternative DNA repair pathway deficiencies.
Researchers at Nerviano Medical Sciences ( Italy ) have reported preclinical characterization of NMS-P648, a selective PARP-1/-3 inhibitor, with a dissociation constants ( KD ), of 0.9 nM on PARP-1 and with greater than four hundred-fold less affinity for PARP-2, as assessed by direct binding assay.
In cells, NMS-P648 inhibits hydrogen peroxide induced poly ADP-ribose ( PAR ) synthesis with an IC50 of 1 nM, confirming expected mechanism of action and indicating that inhibition of PARP-2 is not required for this process.
NMS-P648 has favourable ADME ( Absorption, Distribution, Metabolism, and Excretion ) properties, including stability in liver microsome incubation assays, low efflux ratio in the CACO2 assay and an excellent pharmacokinetic profile, with complete oral bioavailability and exposure levels that increase proportionally with dose.
Testing NMS-P648 on a panel of more than 100 human tumor cell lines using a 2D colony forming assay format revealed that 7 cell lines were highly sensitive to the compound ( IC50 less than 0.5 mcM ).
As expected for an inhibitor of DNA repair, NMS-P648 induced S-phase cell cycle arrest and the formation of nuclear gammaH2AX foci in sensitive cell lines, suggesting the induction of stalled or damaged replication forks, followed by cell death.
Responsive lines were of diverse tissue origin and included BRCA mutated lines, as well as lines resistant to standard chemotherapeutic agents and to known PARP 1/2 inhibitors, indicating a unique activity profile.
The discovery that potent PARP-1/-3 dual inhibitors such as NMS-P648 possess a unique sensitivity profile provides an innovative opportunity in the field of therapeutic intervention in DNA repair processes. ( Xagena )
Source: American Association for Cancer Research ( AACR ), 2015