Fig. 5: Cyclin A/B RxL inhibitors exhibit on-target anti-tumour activity in SCLC mouse models in vivo.
From: Targeting G1–S-checkpoint-compromised cancers with cyclin A/B RxL inhibitors
a,b, Tumour volume curves of NCI-H69 (a) and NCI-H1048 (b) xenografts in athymic nude mice treated with vehicle or cyclin A/B RxL inhibitor (CIRc-028) dosed at 100 mg per kg (mpk) intravenously (i.v.) once per day for 14 days. n = 10 mice per arm (a), and n = 10 (vehicle) and n = 8 (CIRc-028) mice (b). Data are mean ± s.e.m. Statistical analysis was performed using two-way analysis of variance (ANOVA); P < 0.0001. c,d, Representative IHC analysis of p-KNL1 (c) and cleaved caspase-3 (d) in NCI-H69 tumours 18 h after treatment. e,f, Quantification of c (e) and d (f). n = 5 tumours from independent mice per group. Data are mean ± s.d. Statistical significance calculated using unpaired two-tailed Student’s t-tests. g, Schematic for DFCI-393 and DFCI-402 PDX studies with oral cyclin A/B RxL inhibitor CIRc-014. h, mRNA expression of SCLC transcription factors and neuroendocrine markers by RNA-seq. i,j, Tumour volume curves of DFCI-393 (i) and DFCI-402 (j) PDXs treated with CIRc-014 (100 mg per kg orally three times per day (i) or twice per day (j)) or vehicle for 28 days. The dashed lines indicate the start and end of treatment. For i, n = 10 (vehicle) and n = 9 (CIRc-014) mice. For j, n = 10 mice per arm. Data are mean ± s.e.m. Statistical analysis was performed using two-way ANOVA; P < 0.0001. k,l, Representative IHC analysis of p-KNL1 (k) and cleaved caspase-3 (l) in DFCI-393 tumours treated with CIRc-014 (100 mg per kg orally three times per day) for 4 days. m,n, Quantification of k (m) and l (n). For m and n, n = 5 (vehicle) and 6 (CIRc-014) tumours from independent mice. Data are mean ± s.d. Statistical significance was calculated using unpaired two-tailed students t-tests. o, The proposed mechanism: in cancers with compromised G1–S checkpoint (for example, RB1 and TP53 loss) and consequently high E2F activity, cyclin A binds to activating E2Fs through RxL motifs to dampen E2F activity, and cyclin B binds to MYT1 to restrain CDK1 activity during G2. Cyclin A/B RxL inhibitors disrupt both interactions: (1) blocking cyclin A–E2F1 repression increases replication stress; (2) blocking cyclin B–MYT1 binding allows cyclin B to form a neomorphic complex with CDK2, driving RxL-independent phosphorylation (of, for example, stathmin), SAC activation and mitotic death. The diagrams in g and o were created with BioRender. Scale bars, 50 µm (c,d), and 100 µm (×20) and 50 µm (×40) (k,l).
