The T-DXd is arguably the most successful ADC

Discovery and optimization of innovative connector-payloads

Camptothecin (CPT) analogues were selected as cytotoxic payloads

Table 1.

Compared to the other ADCs in Table 1, the simplest structure of the ADC Entry 4 shows low cytotoxicity due to the steric hindrance of the drug fraction and high levels of aggregation

The payload metabolite released by ADC Entry 3 contains primary amino groups derived from a partial linker in the structure, so its membrane permeability is impaired

Link sub-payload system key properties

T-DXd is a new ADC coupled to a TOP1 inhibitor with seven key characteristics that distinguish it from traditional ADCs by linking subloads (Figure 1

Figure 1.

The novel exatecan derivative DXd inhibits TOP1 1 10 times more

Despite the high DAR, the pharmacokinetic curve of T-DXd in cynomolgus monkeys is near-perfect

By conducting plasma stability studies of T-DXd in several species in vitro, it was found that T-DXd exhibited high stability in plasma of all species, including cynomolgus monkeys

The linker between the antibody and the drug fraction contains an enzyme cleavable peptide (GGFG) and an aminomethyl fraction, which reduces hydrophobicity and provides stability

Non-clinical studies

• Antitumor efficacy in non-clinical studies

In mouse transplant models with different HER2 expressions, the efficacy of T-DXd was compared with T-DM1 and lower DAR HER2 ADC (DAR 3.

Figure 6.

• Contribution of bystanders to antitumor effects

Due to the membrane permeability of the released cytotox payload, the released DXd remains killer not only in target cells, but also in neighboring tumor cells (Figure 7

Figure 7.

Xeno-tumor tissues were mixed with NCI-N87 (HER2-positive cell line) and MDA-MB-468-Luc (HER2-negative cell lines) to establish a xenograft mouse model to confirm the bystander antitumor effects

In the T-DM1 treatment group, immunohistochemical analysis showed that HER2-positive cells were cleared, while HER2-negative cancer cells accounted for a large portion of
the tumor tissue.

In the T-DXd treatment group, HER2-positive and HER2-negative cells almost all disappeared, with little or no cancer cell residue in the tissues (Figure 9
).

These results clearly show that under co-inoculation conditions, T-DXd has anti-tumor activity not only for HER2-positive tumor cells, but also for HER2-negative tumor cells, while T-DM1 has no effect
on HER2-negative tumor cells.

Therefore, T-DXd has a bystander anti-tumor effect and is expected to show a higher therapeutic effect
in heterogeneous tumors.

Figure 9.
Immunohistochemical study of HER2 in co-inoculated xenomeric mouse models

• Non-clinical safety assessment

T-DXd was well
tolerated in both rats of non-cross-reactive species and monkeys of cross-reactive species.

When the dose of T-DXd was 20 to 197 mk in rats and 10 to 78.
8 mpk in monkeys, once every 3 weeks, for
a total of 3 times.

In rats, the severe toxic dose for 10% of animals > 197 mpk, and the highest non-severe toxic dose (HNSTD) for monkeys was 30 mpk (Table 2
).

Table 2.
Toxicity studies of repeated administration in rats and monkeys

The results of these non-clinical studies provide strong evidence
for the powerful anti-tumor activity of T-DXd as well as good pharmacokinetics and safety.

• Non-clinical pharmacokinetic evaluation

Non-clinical pharmacokinetics studied the pharmacokinetics of T-DXd and DXd in cynomolgus monkeys
.

After intravenous injection, the blood concentration of T-DXd is similar
to the downward trend of blood concentration of total antibodies.

This indicates that the connecton is stable in plasma and that systemic DXd exposure is low
.

Biological distribution analysis showed that intact T-DXd was mainly present in the bloodstream without tissue-specific retention
.

After intravenous administration of radiolabeled T-DXd, the main route of excretion of DXd is feces
.

The only undimbolized DXd detectable in urine and feces (Figure 10
).

Figure 10.
Biological distribution of T-DXd in cynomolgus monkeys

In summary, the connector exhibits a high degree of stability in the loop, and the high clearance of the DXd when released results in low system exposure
.

In addition, minimal tissue-specific retention and rapid excretion of DXd into the feces in its non-metabolic form have limited potential effects on drug-drug interactions and are also key factors
in the pharmacokinetics of T-DXd.

Clinical studies

Between 10% and 20% of breast cancer patients are HER2 positive
.

Tumors in these patients tend to grow and spread more aggressively
.

Therefore, a number of drugs
have been developed against HER2.

Trastuzumab, a humanized anti-HER2 IgG1 monoclonal antibody, is the first epoch-making antiher2 drug
to significantly improve patient survival.

However, although patients with metastatic disease have a good antitumor response when combined with trastuzumab with common chemotherapy drugs, most patients eventually show signs of
disease progression.

T-DM1 significantly prolonged PFS and OS in patients with HER2-positive advanced breast cancer previously treated with trastuzumab and taxane, and was less toxic than lapatinib plus capecitabine
.

However, her2-targeted therapy for third-line or more treatment routes for HER2-positive metastatic breast cancer has not been established, including T-DM1-resistant tumors
.

In non-clinical studies, T-DXd has been shown to be effective in inhibiting the growth of her2-high- and low-expression tumors, including those that are resistant to T-DM1
.

In addition, non-clinical safety is good
.

Therefore, a phase I clinical dose escalation study was conducted to investigate the safety and tolerability of T-DXd, as well as key studies
of advanced HER2-positive metastatic breast and gastric cancers.

• Phase I dose escalation studies

In the dose-escalation study (NCT02564900) of the Phase I trial, 41 patients received an initial intravenous dose of 0.
8 to 8.
0 mpk (N=3,0.
8, 1.
6, 3.
2, and 8.
0 mpk; N =6, 5.
4 and 6.
4 mpk), dose limiting toxicity (DLT) was assessed
on a 21-day cycle.

There is no DLT and no single dose of MTD is reached
.

The most common adverse effects are mild or moderate gastrointestinal and hematototoxicity
.

Pharmacokinetic profile analysis showed that the pharmacokinetic curve at all doses was nonlinear, with higher doses having a longer half-life, which is consistent
with known HER2-targeted antibodies.

Plasma concentrations of complete T-DXd, released DXd, and total antibodies after treatment with 6.
4 mpk T-DXd are shown in
Figure 11.

At all time points of the assessment, the total antibody concentration was similar to the T-DXd concentration, and only a fairly low concentration of DXd was observed, indicating that the connecton payload had high stability in the patient's in vivo circulation, similar
to non-clinical outcomes.

Figure 11.
In the phase I trial, the dose was 6.
4 mpk, and the average blood concentration-time curve of T-DXd, DXd, and total antibodies in cycles 1 to 3

On the basis of weighing efficacy and safety, in the dose expansion part of the Phase I study, doses of T-DXd monotherapy 5.
4 and 6.
4 mpk were selected for further study
.

T-DXd is used for HER2-positive metastatic breast cancer treated with T-DM1, HER2-positive stomach cancer (GC) treated with trastuzumab, breast cancer with low HER2 expression, her2 expression, or other solid tumors
with HER2 mutations.

Based on the interim analysis of Phase I (Table 3), the latest subgroup analysis of 99 patients with HER2-positive metastatic breast cancer who received trastuzumab and T-DM1 pre-treatment showed that 54.
5% (54/99) confirmed ORR and 93.
9% (93/99) of DCR
.

In addition, a recent subpopulation analysis of 34 patients with HER2-low expression HER2-positive metastatic breast cancer showed that 50.
0% (17/34) of patients with ORR and 85.
3% (29/34) of DCR
were confirmed after T-DXd treatment.

In 44 patients with HER2-positive gastric cancer or gastric-esophageal junction adenocarcinoma who received trastuzumab combined chemotherapy, the ORR of T-DXd was 43.
2% (19/44) and the DCR was 79.
5% (35/44).

Table 3.
T-DXd studies efficacy results in phase I dose-escalation portions by cancer type

In addition, confirmed responses have also been observed in a range of other solid tumors with HER2 expression or HER2 mutations, including non-small cell lung cancer (NSCLC), colorectal cancer (CRC), and salivary gland cancer
.

In all 241 patients, the most common adverse reactions were in the gastrointestinal tract or blood system (Table 4
).

Overall, T-DXd has acceptable security
.

Table 4.
The most common treatment in phase I studies – emergency adverse events

• A key clinical study for HER2-positive breast cancer

The Phase II Destiny-Breast01 Pivot Study (NCT03248492) evaluated adult patients
with HER2-positive metastatic breast cancer who had been treated with T-DM1.

The first part of the study evaluated three different doses of T-DXd to determine the recommended dose (5.
4, 6.
4 and 7.
4 mpk); The second part evaluates the efficacy and safety
of the recommended dose (5.
4 mpk).

184 patients who had previously undergone median 6 treatments received 5.
4 mpk of T-DXd (once every 3 weeks) with an ORR of 60.
9%.


The median effective time was 14.
8 months and the median PFS was 16.
4 months
(Table 5).

Table 5.
Clinical primary endpoint assessment of the Phase II Destination-Breast01 key study

During the study period, the most common grade 3 or higher adverse reactions were a decrease in neutrophil count (20.
7%), anemia (8.
7%), and nausea (7.
6%)
.

Trial drugs accounted for 13.
6% of patients associated with interstitial lung disease (ILD) (grade 1 or 2, 10.
9%); Level 3 or Level 4, 0.
5%; Level 5, 2.
2%) (Table 6
).

Table 6.
Adverse events in the Phase II Destination-Breast01 study

These results show that T-DXd has long-lasting antitumor activity
in the population of pre-treated patients with HER2-positive metastatic breast cancer.

In addition to nausea and bone marrow suppression, ILD was observed, so close monitoring of patients' lung symptoms
was required.

• A key clinical study of HER2-positive stomach cancer

The Phase II DISTY-GASTEC01 key study (NCT03329690) evaluated T-DXd compared with chemotherapy in patients with HER2-positive advanced gastric cancer
.

Patients who developed progressing HER2-positive gastric or gastroesophageal junction adenocarcinoma at at least two previous treatments, including trastuzumab, were randomly assigned to receive T-DXd or chemotherapy
in a 2:1 ratio.

Of the 187 patients treated, 125 received T-DXd and 62 received chemotherapy (55 received irinotecan and 7 received paclitaxel
).

The ORR in the T-DXd treatment group was 51%, compared to 14% in the doctor-selected chemotherapy group (Table 7
).

Overall survival (OS) The T-DXd group was four months (12.
5 vs.
8.
4 months)
than the chemotherapy group.

Table 7.
Evaluation of the primary clinical endpoint of the Phase II DISTY-GASTEC01 key study

The most common adverse events of grade 3 or higher were a decrease in neutrophil count (51% and 24% in the T-DXd group and the doctor-selected group, respectively), anemia (38% and 23%, respectively), and a decrease in white blood cell count (21% and 11%, respectively) (Table 8
).

A total of 12 patients had T-DXd-associated ILD or pneumonia (grade 1 or 2 in 9 patients and grade 3 or 4 in 3 patients
).

One treatment-related death (pneumonia)
was found in the T-DXd treatment group.

Table 8.
Adverse events in the Phase II DISTY-GASTEC01 study

These results show that T-DXd therapy significantly improves response and overall survival in
patients with HER2-positive gastric cancer compared with standard therapy.

The toxic side effects of bone marrow suppression and ILD should be taken seriously
.

Summary

T-DXd as a proof-of-concept study established a new ADC connector payload system that demonstrated superior efficacy and acceptable safety
by using the unique TOP1 inhibitor DXd and stable connectors.

Despite the high DAR8 with hydrophobic payload, the optimized connector produces an ADC with excellent physico-chemical and pharmacokinetic properties, and there is no inclusion of hydrophilic masking parts in the linker structure, such as polyethylene glycol – an unprecedented achievement
.

T-DXd became the first FDA-approved ADC
with an average DAR of about 8.

The success of T-DXd proves the great potential of the new generation of ADC drugs, brings unlimited imagination to everyone, and opens up another wave of ADC research and development boom
.

Congratulations to the T-DXd research team, but also to all those who
have worked quietly on the road to the development of new tumor drugs.

T-DXd is not an end, but a new beginning
.

With more breakthroughs in the future, we believe that cancer has become a chronic disease, and it is
getting closer and closer.

References

1.
Chemical Linkers in Antibody–Drug Conjugates (ADCs).

2.
Trastuzumab Deruxtecan in Previously TreatedHER2-Low Advanced Breast Cancer.

3.
Trastuzumab Deruxtecan: Changing the Destiny of HER2 Expressing Solid Tumors