Abstract
The coadministration of thymidine (dThd) with either l,3-bis(2-chloroethyl)-l-nitrosourea (BCNU) or 3'-|3-(2-chloroethyl)-3-nitrosoureido|-
3'-deoxythymidine (3'-CTNU) to L1210-bearing mice significantly en
hanced the antitumor activity of both nitrosoureas (T-S. Lin and W. H.
Prusoff, Cancer Res., 47:394-397, 1987, and T-S. Lin, P. H. Fischer, J.
C. Marsh, and VV.H. Prusoff, Cancer Res., 42: 1624-1629, 1982). As a
possible mechanism for this observed enhancement, we have investigated
the role of dThd as an inhibitor of poly(ADP-ribose) polymerase
(ADPRP), an enzyme which is activated in response to DNA damage.
Exposure of L1210 cells in culture to 50 MMBCNU resulted in a >10-
fold increase in ADPRP activity within 3-4 h. The polymerase activity
increased with increasing BCNU concentration after a 4-h exposure,
reaching apparent saturation at 50 MMBCNU. However, this activation
was abolished by 2 HIMdThd. Median inhibition of the ADPRP activity
elicited by 30 and 75 MMBCNU occurred at 38 and 135 MMdThd, respectively. When BCNU was replaced by 3'-CTNU, no activation of
ADPRP was observed, even at or above concentrations of 3'-CTNU
previously shown to cause DNA damage. 3'-Amino-3'-deoxythymidine,
the principal hydrolysis product of 3'-CTNU, was found to be an inhibitor
of BCNU-stimulated ADPRP activity with potency similar to dThd.
Furthermore, intact 3'-CTNU was found to inhibit BCNU-stimulated
ADPRP activity. Although 3'-CTNU should be capable of activating
ADPRP by causing DNA damage, our results suggest that no net
activation is observed due to inhibition by the various thymidine species
present. Thus, inhibition of ADPRP by dThd following DNA damage by
BCNU is consistent with the potentiation of antitumor activity previously
reported. However, the observed potentiation of 3'-CTNU activity by
dThd does not appear to result from such a mechanism.