An open-label, randomized, multi-center phase III clinical study: Aim to evaluate the efficacy and safety of FCN-159 monotherapy versus the treatment by investigator's choice in patients with pediatric low-grade glioma harboring KIAA1549-BRAF fusion or BRAF V600E mutation

Inclusion Criteria:

1. Pediatric patients aged between ≥ 2 years and < 18 years; regardless of male or female.
2. Histologically and/or cytologically confirmed diagnosis of low-grade glioma (pLGG diagnosis as Grade 1 or 2 according to the 2021 WHO classification of CNS).
3. KIAA1549-BRAF fusion or BRAF V600E mutation-positive.
4. Patients requiring systemic therapy as determined by the investigator, including patients having disease recurrence or progression, or residual disease of surgery, or unresectable.
5. At least one intracranial measurable lesion that can be reproducibly measured in two dimensions on T2-FLAIR, with the minimum size of the bi-perpendicular diameter of ≥ 10 mm, and can be visible on two or more imaging slice.

6. Karnofsky performance score or Lansky performance score ≥ 70. 7．Adequate organ function within 14 days before enrollment.

Exclusion Criteria:

1. Patients who have previously received any of the following treatments:

   1. Patients who have received chemotherapy drugs or traditional Chinese medicines or herbals with definitive anti-tumor treatment within 4 weeks preceding the first dose of investigational drug;
   2. Patients who have received growth factors that promote platelet or leukocyte count or function within 14 days preceding the first dose of investigational drug;
   3. Patients who received radiotherapy, surgery or immunotherapy within 4 weeks preceding the first dose of investigational drug;
   4. Patients who have participated in other interventional clinical trials within 4 weeks before receiving the first dose of investigational drug;
   5. Patients who have received live vaccines within 4 weeks preceding the first dose of investigational drug, or patients who have received inactivated vaccines and mRNA vaccines within 14 days preceding the study treatment;
   6. Patients who have previously received any other MEK 1/2 inhibitors such as Selumetinib or BRAF inhibitors such as Dabrafenib.

2. Patients with high-grade gliomas, as well as schwannoma, subependymal giant cell astrocytoma (tuberous sclerosis), and diffuse intrinsic pontine gliomas (even if the histological diagnosis is WHO Grade 1 or 2).

3. Patients who require endotracheal intubation for assisted ventilation or tracheotomy should be excluded.

4. Patients who have uncontrollable epilepsy as assessed by the investigator.

5. Patients with dysphagia, active GI diseases, malabsorption syndrome, or other conditions that will interfere with the absorption of the investigational drug.

6. Patients with clinically significant active bacterial, fungal or viral infections, including hepatitis B virus surface antigen positive and hepatitis B virus DNA exceeding 1000 IU/ml. Hepatitis B carriers are allowed to be enrolled. Patients with positive hepatitis C virus (HCV) antibody test; those who have confirmed human immunodeficiency virus (HIV) infection, and are unwilling to undergo HIV testing.

7. Patients with history or current evidence of retinal vein obstruction (RVO), retinal pigment epithelial detachment (RPED), central retinal vein occlusion, glaucoma, and other significant abnormalities in ophthalmological examinations.

8. Interstitial pneumonia, including clinically significant radiation pneumonitis.

9. Grade 3 creatine phosphokinase increased (>5 × ULN - 10 × ULN).