Novel cytotoxic chelators that bind iron(II) selectively over zinc(II) under aqueous aerobic conditions

Abstract

To achieve cellular iron deprivation by chelation, it is important to develop chelators with selective metal-binding properties. Selectivity for iron has long been the province of certain oxygen-donor chelators such as desferrioxamine, which target Fe(III) and exploit the strength of a relatively ionic Fe(III)-O interaction. We have been studying novel chelators that possess mechanisms to selectively chelate +2 biometals, particularly tachpyr [N,N',N"-tris(2-pyridylmethyl)-1,3,5-cis,cis-triaminocyclohexane] and derivatives from N,N',N"-trialkylation and pyridine ring alkylation. Metal-exchange and metal-binding competition reactions have been conducted at pH 7.4, 37 degrees C and time periods until no further change was observed (generally 24-48 h). Under anaerobic conditions, tachpyr is strongly selective for iron, binding 95+/-5% Fe(II) versus 5+/-5% Zn(II) in the forms [Fe(tachpyr)](2+) and [Zn(tachpyr)](2+) respectively. Under aerobic conditions, tachpyr complexes Fe(II) more effectively than Fe(III), forming iminopyridyl complexes [Fe(tachpyr-ox-n)](2+) (n=2, 4) by O(2)-induced and iron-mediated oxidative dehydrogenation. Complexes [Fe(tachpyr-ox-n)](2+) are also strongly bound forms of iron that are unaffected by an excess of Zn(II) (75 mol zinc:1 mol iron complex). The preference of tachpyr for iron over zinc under aerobic conditions appears to be hindered by oxidation of Fe(II) to Fe(III), such that the proportions bound are 44+/-10% Fe(II) versus 56+/-10% Zn(II), in the respective forms [Fe(tachpyr-ox-n)](2+) and [Zn(tachpyr)](2+). However, upon addition of the reducing agent Na(2)S(2)O(4) that converts Fe(III) to Fe(II), the binding proportions shift to 76+/-10% Fe(II) versus 24+/-10% Zn(II), demonstrating a clear preference of tachpyr for Fe(II) over Zn(II). Iron(II) is in the low-spin state in [Fe(tachpyr)](2+) and [Fe(tachpyr-ox-n)](2+) (n=2, 4), which is a likely cause of the observed selectivity. N-methylation of tachpyr [giving (N-methyl)(3)tachpyr] results in the loss of selectivity for Fe(II), which is attributed to the steric effect of the methyl groups and a resulting high-spin state of Fe(II) in [Fe(N-methyl)(3)tachpyr)](2+). The relationship of chelator selectivity to cytotoxicity in the tach family will be discussed.

The Evolution of Iron Chelators for the Treatment of Iron Overload Disease and Cancer

“…8) is a hexadentate chelator based on a cis,cis-1,3,5-triaminocyclohexane scaffold that uses three pyridyl nitrogens and three secondary amine nitrogens to bind iron (for reviews, see Buss et al, 2003cBuss et al, , 2004a. Under aerobic conditions tachpyridine was found to bind Fe(II) more effectively than Fe(III) (Planalp et al, 2002). In addition, tachpyridine has been shown to bind Fe(III) and subsequently reduce it to the Fe(II) state .…”

The Role of Iron Chelation in Cancer Therapy

“…In the presence of oxygen, Fetachpyridine is oxidized to its imino forms [157]. Although Zn 2+ bound tachpyridine more rapidly than Fe 2+ in vitro, Fe 2+ nearly completely displaced Zn 2+ from tachpyridine, indicating preference of tachpyridine for Fe 2+ [159]. Tachpyridine inhibits H 2 O 2 -induced DNA single-strand breaks in vitro, but has no effect on DNA damage caused by the combination of H 2 O 2 and superoxide ion [157].…”

Steric effects caused by N-alkylation of the tripodal chelator N,N′,N″-tris(2-pyridylmethyl)-cis,cis-1,3,5-triaminocyclohexane (tachpyr): structural and electronic properties of the Mn(ii), Co(ii), Ni(ii), Cu(ii) and Zn(ii) complexes

“…Moreover, [Fe((N-Me) 3 tachpyr)] 2ϩ is a complex of high-spin (HS) Fe() that decomposes in anaerobic aqueous pH 7.4 media, whereas [Fe(tachpyr)] 2ϩ is indefinitely stable. 10 The findings indicated that the metal-binding properties of tachpyr are weakened by N-alkylation, likely from the steric effects of the introduced alkyl groups.…”