Prodrug
treosulfan, originally registered for treatment of ovarian
cancer, has gained a use in conditioning prior to hematopoietic stem
cell transplantation. Treosulfan converts nonenzymatically to the
monoepoxide intermediate (EBDM), and then to (2S,3S)-1,2:3,4-diepoxybutane (DEB). The latter alkylates DNA
forming mainly (2′S,3′S)-N-7-(2′,3′,4′-trihydroxybut-1′-yl)guanine
(THBG) and (2S,3S)-1,4-bis(guan-7′-yl)butane-2,3-diol
cross-link (bis-N7G-BD) via the intermediate epoxide adduct (EHBG).
It is believed that DNA cross-linking by DEB is a primary mechanism
for the anticancer and myeloablative properties of treosulfan, but
clear evidence is lacking. Recently, we have proved that EBDM alkylates
DNA producing (2′S,3′S)-N-7-(2′,3′-dihydroxy-4′-methylsulfonyloxybut-1′-yl)-guanine
(HMSBG) and that free HMSBG converts to EHBG. In this paper, we investigated
the kinetics of HMSBG, bis-N7G-BD, and THBG in DNA in vitro to elucidate the contribution of EBDM and DEB to treosulfan-dependent
DNA–DNA cross-linking. Calf thymus DNA was exposed to (A) 100 μM treosulfan, (B) 200 μM
treosulfan, and (C) DEB at a concentration 100 μM,
exceeding that produced by 200 μM treosulfan. Following mild
acid thermal hydrolysis of DNA, ultrafiltration, and off-line HPLC
purification, the guanine adducts were quantified by LC–MS/MS.
Both bis-N7G-BD and THBG reached highest concentrations in the DNA
in experiment B. Ratios of the maximal concentration
of bis-N7G-BD and THBG to DEB (adduct C
max/DEB C
max) in experiments A and B were 1.7–3.0-times greater than in
experiment C. EHBG converted to the bis-N7G-BD cross-link
at a much higher rate constant (0.20 h–1) than EBDM
and DEB initially alkylated the DNA (1.8–3.4 × 10–5 h–1), giving rise to HMSBG and
EHBG, respectively. HMSBG decayed unexpectedly slowly (0.022 h–1) compared with the previously reported behavior of
the free adduct (0.14 h–1), which revealed the inhibitory
effect of the DNA environment on the adduct epoxidation to EHBG. A
kinetic simulation based on the obtained results and the literature
pharmacokinetic parameters of treosulfan, EBDM, and DEB suggested
that in patients treated with the prodrug, EBDM could produce the
vast majority of EHBG and bis-N7G-BD via HMSBG. In conclusion, EBDM
can produce DNA–DNA lesions independently of DEB, and likely
plays a greater role in DNA cross-linking after in vivo administration of treosulfan than DEB. These findings compel revision
of the previously proposed mechanism of the pharmacological action
of treosulfan and contribute to better understanding of the importance
of EBDM for biological effects.