Metal-ion binding properties of (S)-1-[3-hydroxy-2-(phosphonomethoxy)propyl]cytosine (HPMPC, Cidofovir). A nucleotide analogue with activity against DNA viruses

The acyclic nucleoside phosphonate (S)-1-[3-hydroxy-2-(phosphonomethoxy)propyl]cytosine (HPMPC; Cidofovir) has been approved for clinical use in antiviral therapy. We determined the acidity constants of H2(HPMPC)±, as well as that of the nucleobase-free (hydroxy-2-(phosphonomethoxy)propane (H(HPMP)−) (I=0.1M, NaNO3; 25°C). Given that in vivo nucleotides and their analogues participate in reactions typically as metal ion (M2+) complexes, the stability constants of the M(H;HPMPC)+, M(HPMPC), and M(HPMP) complexes with M2+=Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ were measured. Comparisons between results for HPMP2− and previous data for PME2− (CH3CH2OCH2PO32-; phosphonomethoxyethane) revealed the hydroxyl-group effect. The hydroxyl group stabilizes only complexes with the heavier alkaline earth metal ions (Ca2+, Sr2+, Ba2+). For all other complexes, the enhanced stability can solely be explained by the formation of 5-membered chelates involving the ether oxygen; these occur in equilibrium with simple 'open' phosphonate-M2+ species. The stability of the M(HPMPC) complexes is also higher than expected for a phosphonate-only coordination, indicating that chelates are formed, but comparison with the HPMP2− data shows that the cytosine base does not affect complex stability. Similar observations were made previously with related cytosine derivatives. The stability data for the monoprotonated M(H;HPMPC)+ complexes suggest that these carry H+ predominantly on the phosphonate group, and M2+ on the nucleobase.