**5.2. Combinatory mutations**

Mutations in *ACVR1* gene activate the *ALK2* receptor, increase phosphorylation of SMAD proteins, and up-regulate genes in BMP developmental signaling pathway. These mutations are also described in patients with fibrodysplasia ossificans progressiva (FOP), although amino acid substitutions that occur in DIPGs have not been found in FOP patients [51]. Because FOP is not associated with cancer predisposition, it is likely that *ACVR1* mutations provide a selective advantage in the presence of other critical partner mutations, rather than driving

Pathways common in a variety of tumor types occur also in DIPG. One example is *TP53* checkpoint, harmed by *TP53* mutations, which occurs in approximately 55% of patients with HGGs and is associated with *H3F3A*, *ATRX*, and *DAXX* mutations [45]. Mutant p53 proteins have an extended half-life and can be detected by immunohistochemistry (IHC) due to their protein accumulation [53]. In 9–23% of DIPGs, there are also mutually exclusive mutations in the *TP53* target gene *PPM1D*, which plays a role downstream of p53 in the DNA damage

Also, the association of p53 abnormalities in the context of *PDGFRA* amplification or *PI3K* mutations raises the possibility that the *PI3K* signaling pathway constitutes a major compo‐ nent of the pathogenesis of DIPG [49]. *PDGFRA* is known to be expressed in malignant gliomas and plays a role during embryonic development, suggesting an embryonic origin for DIPG,

Other important genomic alterations include ATRX, TERT, MYCN, and PTEN. The identifi‐ cation of driver mutations in DIPG helps more than to confirm the diagnosis at a molecular level: it provides relevant clinical and prognostic information, leading to the improvement in the genomic and epigenetic knowledge of DIPG. The combination of different mutations in a singular patient elucidates the fact that DIPG is a complex and varied pathology comprised of different molecular subgroups that share the same clinical features as well as a grim prognosis.

The infiltrative nature of DIPG makes effective therapy extremely difficult and characterizes this tumor as one of the most, if not the most challenging childhood cancer. The successful and efficient delivery of effective therapy to the DIPG tumor is the major challenge that contrib‐ utes to the poor treatment options in children with this disease. For the effectiveness of a therapy, the agent needs to have certain important characteristics: (A) it has to be active and reach its molecular target in the tumor cell in adequate concentrations, (B) it has to reach the target for an adequate amount of time, and (C) the tumor cells need to be sensitive to the compound. Many factors can affect the level of a drug in the brain tumor site, including the concentration of this drug in the bloodstream, amount of protein to tissue binding, and the degree of central nervous system (CNS) penetration [8]. In DIPG, the BBB is often intact and has the ability to restrict the delivery of chemotherapeutic agents. Even for drugs that are

**5. Challenges in the treatment: BBB and combinatory mutations**

given the incidence of this disease in middle childhood [55].

tumor initiation [52].

410 Neurooncology - Newer Developments

response [51,54].

**5.1. The blood–brain barrier**

Novel genome-wide studies and increasing availability of tumor tissue, from autopsy and surgical biopsy samples, show that each individual tumor harbors multiple mutations, as well as copy number abnormalities, gene expression, and methylation patterns. While there are several ongoing clinical trials using target therapies, targeting only specific mutations in a patient has rarely been effective. Studies are showing that using chemotherapy alone or in combination with RT does not lead to any additional survival benefit [8,10,11].

In this context, multi-targeting combinatory regimens are the new promise for DIPG. Considering that DIPG is not a single disease and that HGGs harbor distinct genomic aberrations compared to adult glioblastomas, the heterogeneity of DIPG can be correlated with age of onset and the range of genomic mutations particular to each subtype. It is also be‐ lieved that DIPG heterogeneity partially accounts for its resistance to current targeted therapies.

The main challenge is to combine different molecularly targeted chemotherapeutics that in a mutual mechanism of action would target the distinct driver mutations of each patient. For this purpose, it is essential to deeply investigate the mechanism of action of these drugs, as well as the pathway of each mutation found in DIPG. Preclinical studies conducted *in vitro* and *in vivo* are crucial to gain a perspective of what can be done in the clinic. Also, it is necessary to discover proper drug concentrations, study the ability of the agent to overcome the BBB, and minimize the possible adverse effects.

Hopefully, a better understanding of the molecular landscape of DIPG patients will lead to the use of combinatory therapy not only in preclinical models but also in clinical trials, aiming for an optimal personalized drug combination that can be used in children with DIPG.

#### **5.3. Clinical trials**

Clinical trials are the best way to evaluate treatment for DIPG and to test if the therapeutic agents are effective or not. While determining the origin of DIPG is important, it is also essential to evaluate new drug targets, biological agents, and immunotherapeutic strategies in clinical trials to determine if they can be used in the clinic. Numerous molecularly targeted chemo‐ therapeutic agents have been tested in the past year with and without RT (**Table 2**).



**Table 2.** Clinical trials conducted for patients with DIPG in 2015.

Hopefully, a better understanding of the molecular landscape of DIPG patients will lead to the use of combinatory therapy not only in preclinical models but also in clinical trials, aiming for

Clinical trials are the best way to evaluate treatment for DIPG and to test if the therapeutic agents are effective or not. While determining the origin of DIPG is important, it is also essential to evaluate new drug targets, biological agents, and immunotherapeutic strategies in clinical trials to determine if they can be used in the clinic. Numerous molecularly targeted chemo‐

**Chemotherapy Radiotherapy**

No

No

59.4 Gy

drugs

Yes

No

1.8 Gy for 30 treatments over 6–7 weeks. Total dose of radiation 54 Gy

prescription dose of 54–

Standard radiation therapy followed by molecular based therapy with FDA-approved

NCT0242061 Vorinostat; temsirolimus Single daily fractions of

NCT01222754 Lenalidomide Five days per week to a

NCT01688401 Melphalan hydrochloride intra-arterially

NCT02233049 Erlotinib; dasatinib; everolimus

NCT02274987 Individualized treatment

plan for each patient and different approaches depending on the molecular profile of the patient's tumor

(specialized tumor board recommendations)

cyclophosphamide

bevacizumab; irinotecan

NCT01837862 Mebendazole;

NCT01644773 Crizotinib; dasatinib No

an optimal personalized drug combination that can be used in children with DIPG.

therapeutic agents have been tested in the past year with and without RT (**Table 2**).

**trial ID**

Anti PD1 antibody in DIPG NCT01952769 MDV9300 (pidilizumab);

**5.3. Clinical trials**

412 Neurooncology - Newer Developments

DIPG

**Title Clinical**

Study of suberoylanilide hydroxamic acid (SAHA) with temsirolimus in children with

Study of the combination of crizotinib and dasatinib in pediatric research participants

Biological medicine for DIPG eradication

Lenalidomide and radiation therapy in

Molecular profiling for individualized

A phase I study of mebendazole for the treatment of pediatric gliomas

Intra-arterial chemotherapy for the treatment of progressive DIPGs

with DIPG and HGG

(BIOMEDE)

HGGs or DIPGs

treatment plan for DIPG

Although clinical trials are a big hope in finding a treatment that could lead to a better prognosis, it also has its own challenges—not all the patients qualify for participation; the families have a crucial responsibility in the decision to participate or not; the length of time to complete a trial can be long, given the rarity of this cancer; and finally, there are strict guidelines that need to be followed in order to guarantee the patient safety and minimize the risk.

Every time that a clinical trial is formulated, it has great potential, but not a guarantee of benefit. However, it is important to recall the example of so many other pediatric cancers, such as leukemia, lymphoma, and Wilms' tumor, which obtained their success in treatment because of the persistence of the researchers and physicians in clinical trials.

The biggest hope is that in the future, patients can be divided into subgroups according to their genetic, epigenetic, and proteomic molecular particularities through a biopsy of their tumor. This way, targeted therapy could be individualized—however, the only way to achieve this goal is through improvement of research and investment in more clinical trials.
