respective MTDs. The six compounds profiled in this study
all induced extensive tumor necrosis. Within 24 hours of
administration, the entire center of the tumor was necrotic
with a thin viable rim of tumor cells surviving at the
periphery. This pattern of central necrosis with a thin viable
rim of tumor cells was first reported with CA4P (30) and is
consistent with vasculature targeting agents (18, 31–33).
However, when the dose required to induce significant
tumor necrosis was compared with the dose that caused
significant but reversible toxicities, it became evident that
some compounds were more effective than others.
Also observed with this series of compounds was that the
compounds with the most potent antivascular effects were
not the most potent in vitro (4). Indeed, although MX-
126303 had the best in vitro cytotoxic activity, it induced
tumor necrosis at levels close to its MTD, suggesting that its
toxicity is linked with its effect on tubulin inhibition rather
than targeting selectively endothelial cells. The antivascular
activity of MX-126303 is reminiscent of other tubulin-
binding agents, such as colchicine and vinblastine, which
However, its pharmacokinetic profile is similar to classic
antivascular agents such as ZD6126 and CA4P, which have
short plasma half-lives (29, 34). On the other hand, MX-
116407, while less cytotoxic than MX-126303 in vitro, had
more tumor-selective antivascular activity. We observed
80% tumor necrosis at 25 mg/kg, corresponding to one-
fourth of its MTD. This may be related to the differences in
pharmacokinetic profile. Although both MX-116407 and
MX-126303 have rapid distribution phases, resulting in
maximum concentrations at the earliest time point evalu-
ated (10 and 12.3 Ag/mL, respectively), their clearance rates
were quite different, varying by 2.4-fold. This results in a
8-fold difference in concentration after 45 minutes (3.5 ver-
sus 0.44 Ag/mL for MX-116407 and MX-126303, respectively;
Fig. 1). A rapid distribution coupled with a slow clearance
is thus likely to be important for good antitumor activity.
A second explanation might relate to our finding that
apoptosis induction following a 3-hour treatment with MX-
126303 is irreversible after drug washout, whereas induc-
tion of apoptosis following MX-116407 treatment is
reversible (4). This may suggest that MX-116407 and MX-
126303 have different tubulin-binding kinetics, leading to
differences in pharmacologic profile. Indeed, the improved
therapeutic index observed with CA4P compared with
colchicine has been correlated to their differences in the
on/off rate of binding to tubulin (11, 17, 35), leading to the
hypothesis that a successful vascular targeting agent would
have reversible binding kinetics and relatively rapid
clearance in vivo (19).
The vascular targeting activity of this novel series of
compounds, suggested by the induction of tumor necrosis,
led us to investigate their antitumor potential. Whereas
treatment of mice bearing MDA-MB-435 tumor xenografts
with MX-58151, MX-58276, and MX-126303 resulted in poor
antitumor activity with % T/Cs of 65, 75, and 75,
respectively, MX-76747, MX-116214, and MX-116407
resulted in moderate antitumor activity with % T/Cs of
58, 55, and 43, respectively. According to National Cancer
Institute criteria, compounds resulting in % T/C values of
V42 are considered to have antitumor activity (24). The
moderate antitumor activity is likely a result of regrowth of
the tumor originating from the viable rim of tumor cells
remaining after the treatment. We have observed that the
MDA-MB-435 tumors have repopulated spontaneously 14
days after the cessation of treatment, although % T/C
values were calculated at day 50, 10 days after cessation of
treatment. We have further evaluated MX-76747, MX-
116214, and MX-116407 in other human tumor xenograft
models. In the human breast (MX-1) tumor xenograft, all
three compounds showed significant antitumor activity
with % T/Cs ranging between 0.7 and 17. In the human
lung (Calu-6) tumor xenograft, MX-116407 was highly
active, producing tumor regressions in all 10 animals.
Moreover, MX-116407 significantly enhanced the antitumor
activity of cisplatin, producing tumor-free animals in a
significant number (4 of 10) of cases at time of sacrifice.
In summary, we have identified 2-amino-4-(3-bromo-4,5-
dimethoxy-phenyl)-3-cyano-4H-chromenes as a novel se-
ries of vascular targeting agents with promising antitumor
activity. Our encouraging results with MX-116407 strongly
support its continued development as a novel anticancer
agent for human use.
We thank Louis Vaillancourt for the synthesis of ZD6126 and Johanne
Cadieux for assistance in the preparation of the article.
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