In the course of searching potential antitumor agents from the plant or animal drugs, it was found that the root preparation of Scutellaria baicalensis showed a good cytotoxic activity against L1210 cell. Bioassy-combined column chromatographic fractionation resulted in isolation of the pure cytotoxic substances, which proved to be 5,2´-dihydroxy-6,7,8,6´-tetra-methoxyflavone(ED 50=1.7 ㎍/㎖, skullcapflavone II) and 5,6,7-trihydroxy-flavone(ED 50=15.0 ㎍/㎖, baicalein)(1).
The activity of skullcapflavone II was confirmed through synthesis. At the same time we have synthesized a series of the analog flavones and tested their cytotoxicities against the cell(2). Of the flavone synthesized 5,2´,6´-trihydroxy-6,7,8-trimethoxy-(ED 50=4.5 ㎍/㎖) and 5-hydroxy-6,7,8-trimethoxy-2´-benzyloxyflavones(ED50=15.4 ㎍/㎖) are active.
The ratio(logmm/logmm) of the UV-absorbancies of band I to band II of the flavones were corelated to the activities of the flavones. Only the flavones showing the ratio range of 1.073-1.109 were cytotoxic, what means that a certain conformation between the ring C and B of the flavones is required for the cytotoxic activity.
To find an essential structural part for the activity we have modified the structure of baicalein. The 6,7-dimethylated baicalein was hydroxylated per the modified Elb´s persulfate oxidation to obtain 5,8-dihydroxy-6,7-dimethoxyflavone(ED 50=7.5 ㎍/㎖) which was the only active substance among the modified structures.
Under the flavones studied, no structure-activity-relation was observable. However, the presence of the peroxygenated A-ring seems to be necessary for the activity.
In an early work(3) we have found that 7-geranylcoumarin was cytotoxic against L1210 cell. The cytotoxic part of the structure was proven to be geraniol. In order to observe the possible enhancement of the activity, the twin compounds between the cytotoxic flavones and geraniol were synthesized and their cytotoxicities tested(4).
The introduction of the geranyl group to the analogs of skullcapflavone II or to wogonin reduced the activity(ED 50=4.5 ㎍/㎖ of 5,2´,6´-trihydroxy-6,7,8-trimethoxyflavone → ED 50=8.5 ㎍/㎖ of 5,2´-dihydroxy-6,7,8-trimethoxy-6´-geranyloxyflavone) or kept it unchanged(ED 50=17.0 ㎍/㎖ of 5,2´-dihydroxy-6,7,8-trimethoxy-6´-benzyloxyflavone → ED 50=14.0 ㎍/㎖ of 5-hydroxy-6,7,8-trimethoxy-2´-benzyloxy-6´ -geranyloxyflavone, ED 50=15.0 ㎍/㎖ of 5,7-dihydroxy-8-methoxyflavone → ED 50=20.0 ㎍/㎖ of 5 -hydroxy-7-geranlyoxy-8-rnethoxyflavone. In the contrast, the geranylation of the haicalein series enhances the activity(ED 50=15.0 ㎍/㎖ of 5,6,7-trihydroxyflavone → ED 50=2.3 ㎍/㎖ of 5,6-dihydroxy-7-geranyloxyflavone).
Skullcapflavone II was administered intraperitoneally to the S-180 inoculated ICR and L1210 cell inoculated BDFl mice and the prolongation of their life span observed. The same was carried out for baicalein and 7-geranylbaicalein. Intraperitoneal injection of 40 ㎎/㎏ of skullcapflavone II has prolonged the life span of S-180 inoculated ICR mice by 66% and of the L1210 inoculated BDFl mice by 22%, respecively.
10 ㎎/㎏ administration of baicalein and 7-geranylbaicalein has prolonged the life span of S-180 inoculated ICR mice by 26 and 44%, respectively.
The ICR mice were inoculated with 1 × 10 S-180 cells at the left groin. 24hr after the tumor inoculation skullcapflavone II were injected around the inoculation point once a day for successive 7 days. 40 ㎎/㎏ administration have inhibitted the growth of the S-180 solid tumor by 71%. Of 16 mice treated six animals carried no tumor.
(1) S.H. Ryu, B.Z. Ahn and M.Y. Pack, Planta Medica 1985, 355.
(2) S.H. Ryu, B.T. Yoo, B.Z. Ahn and M.Y. Pack, Archiv der Pharmazie(Weinheim) 318. 659(1985).
(3) K.S. Kang, S.H. Ryu and B.Z. Ahn, Arch. Pharm. Res., 8, 187(1985).
(4) K.U. Baik and B.Z. Ahn, Arch. Pharm(Weinheim) 1987 in press.
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