Circadian delivery

Assessing the Efficacy of Circadian Delivery of the Anticancer Drug 5-FU 283... 

It is useful to compare the circadian patterns of 5-FU delivery with the more conventional constant infusion drug-delivery pattern, in which the amount of 5-FU delivered over the 24-h period is the same as for the circadian delivery schedules. The quantity of 5-FU (Qsfu) delivered over 24 h according to the semi-sinusoidal schedule defined by Eq. (3) is given by Eq. (5) ... 

To investigate the effect of the peak time of circadian delivery of 5-FU, we compare in Fig. 10.4a four circadian schedules with peak delivery at 4 a.m., 10 a.m., 4 p.m., and 10 p.m., for a cell cycle variability of 15%. The data on cumulated cell kill by 5-FU indicate a sharp difference between the circadian schedule with a peak at 4 a.m., which is the least toxic, and the other schedules. This difference is even more striking when cells are better synchronized, for smaller values of variability V (data not shown). The most toxic circadian schedules are those with a peak delivery at 4 p.m. or 10 a.m. We compare in Fig. 10.4b the least and most toxic circadian patterns of 5-FU delivery with the continuous infusion of 5-FU. Continuous delivery of 5-FU appears to be slightly more toxic than the circadian pattern with a peak at 4 p.m. 

To clarify the reason why different circadian schedules of 5-FU delivery have distinct cytotoxic effects, we used the cell cycle automaton model to determine the time evolution of the fraction of cells in S phase in response to different patterns of circadian drug administration, for a cell cycle variability of 15%. The results, shown in Fig. 10.5, correspond to the case considered in Fig. 10.4, namely, entrainment of a 22-h cell cycle by the circadian clock. The data for Fig. 10.5a clearly indicate why the circadian schedule with a peak at 4 a.m. is the least toxic. The reason is that the fraction of cells in S phase is then precisely in antiphase with the circadian profile of 5-FU. Since 5-FU only affects cells in the S phase, the circadian delivery of the anticancer drug in this case kills but a negligible amount of cells. 

The cases of peak delivery at 10 a.m. (Fig. 10.5b) or 10 p.m. (Fig. 10.5d) are intermediate between the two preceding cases. Overlap between the peak of 5-FU and the peak of cells in S phase is only partial, but it is still greater in the case of the peak at 10 a.m., so that this pattern is the second most toxic, followed by the circadian delivery centered around 10 p.m. The comparison of the four panels Fig. 10.5a-d explains the results of Fig. 10.4a on the marked differences in cytotoxic effects of the four 5-FU circadian delivery schedules. The use of the cell cycle automaton helps clarify the dynamic bases that underlie the distinctive effects of the peak time in the circadian pattern of anticancer drug delivery. 

The effect of variability on drug cytotoxicity markedly depends on the temporal pattern of 5-FU delivery. When the peak in the circadian delivery of 5-FU occurs at 4 p.m., i.e. when the circadian schedule of 5-FU administration is most toxic to the cells, whether in the absence or presence of entrainment by the circadian clock, cytotoxicity increases as the degree of variability decreases. The effect is more marked in the conditions of entrainment a threshold in cytotoxicity then exists between... 

Numerical simulations therefore indicate that the least damage to the cells occurs when the peak of 5-FU circadian delivery is at 4 a.m., and when cells are well synchronized, i.e., when cell cycle variability V is lowest. In contrast, when the peak of 5-FU circadian delivery is at 4 p.m., cytotoxicity is enhanced when cells are well synchronized. The cytotoxic effect of the drug, therefore, can be enhanced or diminished by increased cell cycle synchronization, depending on the relative phases of the circadian schedule of drug delivery and the cell cycle entrained by the circadian clock. Continuous infusion of 5-FU is nearly as toxic as the most cytotoxic circadian pattern of anticancer drug delivery. 

The study of various anticancer drugs shows that many possess an optimal circadian delivery pattern, according to the phase of the cell cycle in which the cytotoxic effect is exerted. A case in point is provided by 5-FU. This widely used anticancer drug interferes with DNA synthesis and acts during DNA replication, the S phase of the cell cycle. Cells exposed to 5-FU during the S phase have an enhanced proba-... 

The cell cycle automaton model permits us to clarify the reason why circadian delivery of 5-FU is least or most toxic when it peaks at 4 a.m. or 4 p.m., respectively. Indeed, the model allows us to determine the position of the peak in S-phase cells relative to that of the peak in 5-FU. As shown in Fig. 10.5, 5-FU is least cytotoxic when the fraction of S-phase cells oscillates in antiphase with 5-FU (when 5-FU peaks at 4 a.m.) and most toxic when both oscillate in phase (when 5-FU peaks at 4 p.m). Intermediate cytotoxicity is observed for other circadian patterns of 5-FU (when the drug peaks at 10 a.m. or 10 p.m.), for which the peak of 5-FU partially overlaps with the peak of S-phase cells. For the continuous infusion of 5-FU, the peak in S-phase cells necessarily occurs in the presence of a constant amount of 5-FU. Hence, the constant delivery pattern is nearly as toxic as the circadian pattern peaking at 4 p.m. 

The results of simulations indicate that, when the circadian delivery of 5-FU peaks at 4 a.m., differential effects of the drug on a population of healthy cells and on a population of tumor cells may be observed depending on whether the two cell populations are entrained or not by the circadian clock [33]. Another source of differential effect pertains to the degree of variability, given that, as previously noted, synchronization of the cells minimizes cytotoxic damage when the circadian 5-FU modulated delivery pattern peaks at 4 a.m. The results are markedly different when the circadian pattern of 5-FU delivery peaks at 4 p.m. [33]. Then the cytotoxic effect of the drug on the two populations is the inverse as that predicted for the circadian pattern peaking at 4 a.m. The effect of variability therefore depends on the circadian pattern of 5-FU delivery and on the possibility of entrainment of the cell cycle by the circadian clock. 

This time controlled release tablet with a designated lag time followed by a rapid release may provide an alternative to site-specific delivery of drugs with optimal absorption windows or colonic delivery of drugs that are sensitive to low pH or enzyme action for the treatment of localized conditions such as ulcerative colitis, Crohn s disease, and irritable bowel syndrome (IBS). Also, by controlling a predetermined lag time of drug from dosage form, the release behavior can be matched with the body s circadian rhythm pattern in chronotherapy. 

In simulating the cell cycle automaton response to 5-FU, we impose a circadian profile of the anticancer medication similar to that used in clinical oncology [30, 31] 5-FU is delivered in a semi-sinusoidal manner from 10p.m. to 10a.m., with a peak at 4 a.m. (Fig. 10.3b). During the remaining hours of the day and night, the drug concentration is set to zero. For comparison, we consider similar drug delivery patterns shifted in time, with peak delivery either at 10 a.m., 4 p.m., or 10 p.m. 

Circadian 5-FU Administration Effect of Time of Peak Drug Delivery... 

Fig. 10.5 Explanation of the cytotoxic effect of various circadian schedules of5-FU delivery with peak at 4 a.m. (a), 10 a.m. (b), 4 p.m. (c), or 10 p.m. (d), and of continuous 5-FU delivery (e). Data are obtained for variability V = 15% and for a cell cycle duration of 22 h, in the presence of entrainment by the circadian clock. The hatched area shows the fraction of cells in S phase exposed to 5-FU and thus likely marked to exit the cell cycle at the next G2/M transition. The curves in Fig. 10.4 showing the cumulated number of cells killed indicate that the schedule with peak delivery at 4 a.m. is the one that causes minimal damage to the cells because the peak...

The case of the continuous infusion of 5-FU is considered in Fig. 10.5e. Because the total amount of 5-FU administered over 24 h is the same as for the circadian semi-sinusoidal patterns, the level of 5-FU - and hence the cytotoxic effect of the drug - is sometimes below and sometimes above that reached with the circadian schedule. The numerical simulations of the automaton model indicate that the cytoxicity is comparable to that observed for the most toxic circadian pattern, with peak delivery of 5-FU at 4 p.m. 

We compared the effect of the continuous administration of 5-FU with various circadian patterns of 5-FU delivery peaking at 4 a.m, 10 a.m., 4 p.m., or 10 p.m. in the presence of entrainment by the circadian clock, by measuring the normalized, cumulative number of cells killed by 5-FU (Fig. 10.4). Several conclusions can be drawn from this comparison. First, the various circadian patterns of 5-FU delivery have markedly different cytotoxic effects on diurnally active cancer patients the least toxic pattern is that which peaks at 4 a.m., while the most toxic one is that which peaks at 4 p.m. The other two patterns peaking at 10 a.m. or 10 p.m. exert intermediate cytotoxic effects. Conventional continuous infusion of 5-FU is nearly as toxic as the circadian pattern of 5-FU delivery peaking at 4 p.m. 

The results presented here point to the interest of measuring, both in normal and tumor cell populations, parameters such as the duration of the cell cycle phases and their variability, as well as the presence or absence of entrainment by the circadian clock. As shown by the results obtained with the cell cycle automaton model, these data are crucial for using the model to predict the differential outcome of various anticancer drug delivery schedules on normal and tumor cell populations. In a sub-... 

To some extent the idea of resonance is also present in the case of circadian 5-FU delivery. Indeed, the circadian patterns of 5-FU which peak at 4 a.m. or 4 p.m. correspond to oscillations that are, respectively, in antiphase or in corresponding phase with the circadian variation of the fraction of cells in S phase. This effect can be seen even for cell cycle durations that differ from 24 h, because of the entrainment of the cell cycle by the circadian clock. 

Here, as in a previous publication [33], we used the cell cycle automaton model to probe the cytotoxic effect of various patterns of circadian or continuous 5-FU delivery. The results provide a framework to account for experimental and clinical observations, and to help us predict optimal modes of drug delivery in cancer chronotherapy. By explaining the differential cytotoxicity of various circadian schedules of 5-FU delivery, the model clarifies the foundations of cancer chronothera-peutics. In view of its versatility and reduced number of parameters, the automaton model could readily be applied to probe the administration schedules of other types of anticancer medications active on other phases of the cell cycle. 

Levi, F., Focan, C., Karaboue, A., de la Valette, V., Focan-Henrard, D., Baron, B., Kreutz, M.F., Giacchetti, S. Implications of circadian clocks for the rhythmic delivery of cancer therapeutics. Adv. Drug Deliv. Rev. 2007, in press. 

Variable release the delivery system provides drug input at a variable rate, to match, for example, endogenous circadian rhythms, or to mimic natural biorhythms. 

---