Scheduling patterns in which cells were exposed to both drugs, each for 24
h were shown to result in a synergistic response in both SHSY5Y cells and NGP cells when cisplatin was administered prior to etoposide. SHSY5Y cells also exhibited this response when the exposure time to etoposide was reduced to 4
h (NGP cells were not tested with this schedule). Introducing a 24
h drug-free period between drug exposures gave a similar pattern of combination indexes to the other schedules. Overall, the results showed that changing schedules from cisplatin first to cisplatin last resulted in a change from synergy to antagonism. The exact degrees of synergy/antagonism are probably dependent upon both the model underlying the analysis method and the techniques used to measure cytotoxicity.
In the recently developed OJEC protocol for neuroblastoma therapy, carboplatin is administered for 1
h immediately following a 4
h etoposide infusion (Tweddle et al, 2001
), allowing day-care chemotherapy. This is a reversal of the OPEC protocol in which cisplatin is infused for 24
h, followed by 24
h post-hydration and finally 4
h of etoposide. Replacement of cisplatin by carboplatin also reduces toxic side-effects. Cisplatin and carboplatin form essentially the same DNA-adduct structures (Knox et al, 1986
), however, carboplatin monofunctional adducts are converted to bifunctional adducts much more slowly than those formed with cisplatin (Knox et al, 1986
; Peng et al, 1997
), further complicating analysis of synergy. This slow formation of the toxic products with carboplatin will tend to enhance the antagonistic effects of administration of the platinum agent after etoposide.
Etoposide acts via the stabilization of topoisomerase II (topo II) cleavable complexes. This effect is reversible and furthermore etoposide has a short elimination half-life (Hsiang and Liu, 1989
; Slevin, 1991
; Caldecott et al, 1993
; Willmore et al, 1998
). Together with the slow formation of carboplatin-DNA adducts, the swift reversal of etoposide stabilised topo II cleavable complexes may result in reduced anti-tumour effect in patients.
In conclusion, if the results of this study are reflected by the action of drugs in the patient, then the adoption of schedules in which platinum agent follows etoposide and in which formation of platinum-DNA cross-links is delayed (carboplatin as opposed to cisplatin) may result in sub-optimal anticancer action. An understanding of the underlying mechanisms of interaction may help assessment of the clinical relevance of these effects.