Trop2 ADC drug competition: who will be better

Trop2 is a transmembrane glycoprotein and calcium signaling protein that is limited
in normal human tissue.
It is expressed in large quantities in a variety of malignant tumors and participates in a variety of carcinogenic signaling pathways, resulting in tumor occurrence, invasion and metastasis
.
As a result, Trop2 has become an attractive therapeutic target for cancer therapy
.
The Trop2 ADC drug (Trodelvy™, Socituzumab Gegitecan) has been approved for the treatment of metastatic triple-negative breast cancer
.
However, it is unclear whether the success observed in Trop2-positive breast cancer can be replicated in other types of tumors, as there are differences
in Trop2 expression levels and function in different types of cancer.
In addition to Socituzumab Gegitecan, a number of other promising new Trop2 ADC drugs have also entered the clinic and have shown some potential
.
These drugs will likely change the treatment landscape
for Trop2-positive tumors.
This article will make an introduction to these Trop2 ADC drugs that have entered the clinical stage, and make a simple comparison
from several key points of ADCs.

The clinical-stage Trop2 ADC represents the drug

• Sacituzumab govitecan (IMMU-132, SG)

SG is a novel ADC class of drugs developed by Immunomedics and consists of
humanized kappa monoclonal antibody (hRS7) against Trop2, maleimide-peg-acid sensitive cleavage carbonate linkers, and irinotecan metabolite (SN-38).
hRS7 is able to bind directly to Trop2-expressing cancer cells and trigger the internalization of SG, leading to hydrolysis of the CL2A linker and release of the topoisomerase I inhibitor SN-38, thereby inducing DNA damage and eventual apoptosis
.
In addition, in an acidic environment, pH-dependent linkers are hydrolyzed around the tumor, and the release of SN-38 kills surrounding tumor cells
through the bystander effect.
In in vitro studies, SN-38 can prevent FUBP1 from binding to its single-stranded target DNA fusion protein, induce dysregulation of the FUBP1 target gene in human hepatocellular carcinoma, reduce tumor cell proliferation, and promote apoptosis
.
There is also some evidence that the hRS7 antibody of the ADC induces ADCC action
.
In conclusion, preclinical studies have shown that SG has multiple mechanisms
of action that inhibit Trop2-positive tumors.

SG's Phase I/II clinical trial (NCT01631552) was conducted
in previously treated patients with advanced epithelial cancer.
In the Phase I clinical trial, 2 of the 25 patients treated with SG, including TNBC, NSCLC and other cancers, achieved partial remission (PR), while 16 patients were stable
.
The overall safety profile was good, with only 3 cases of fatigue, 2 cases of neutropenia, 1 case of diarrhea, and 1 case
of leukopenia.

In the Phase II dose expansion portion of the NCT01631552 clinical trial, 178 patients with advanced solid tumors were treated
with SG 8 or 10 mg/kg.
The results show that SG is safe
.
In addition, the objective response rate (ORR) and clinical benefit rate (CBR) were higher in the 10 mg/kg dose group than in the 8 mg/kg dose group
.
Therefore, the 10 mg/kg dose was selected as an additional clinical trial
in patients with solid tumors.

After clinical assessment of the overall safe population of the study (OSP=495), the toxicity curve was consistent and predictable
.
SG has antitumor activity
against NSCLC (n=47), SCLC (n=50), mUC (n=45), HR+/HER2−(n=54) BC, and mTNBC (n=108).
In the mTNBC group, the final ORR was 33.
3%, with 33 patients in partial response (PR) and 3 patients in complete response (CR).

PFS was 5.
5 months and median OS was 13.
0 months
.
The median duration to treatment (DOT) (5.
1 months) for SG was approximately twice that
of previous anti-cancer drugs (2.
5 months).
Based on these excellent results, in 2020 the FDA accelerated the approval of SG to treat mTNBC patients
who have received at least two prior medications.

Only refractory or relapsed mTNBC patients participated in the Phase III clinical trial of SG (NCT02574455).

In this clinical trial, SG was compared
to four single-agent chemotherapy selected by doctors (capecitabine, gemcitabine, vinorelbine, and eriblin).
A total of 468 patients were enrolled, of which 235 received SG and 233 received physician-selected treatment (TPC).

The SG-receiving group had an ORR of 35% (4% CR, 31% PR), significantly higher than the TPC-receiving group (5% ORR; 1%CR，4%PR)
。 Patients treated with SG and TPC had median PFS of 5.
6 months and 1.
7 months, respectively, and median OS of 12.
1 months and 6.
7 months
, respectively.
Regardless of their age, patients receiving SG had a clear survival benefit compared with TPC and an acceptable
safety profile.
Based on the results of the confirmatory trial, the FDA approved SG for patients with unresectable locally advanced or mTNBC who have received two or more prior systemic therapies
.
The first-line efficacy of SG in the treatment of mTNBC will be evaluated
in Phase III study ASCENT-03 (NCT05382299).

Unlike the excellent results in TNBC, SG has limited efficacy in HR+/HER2 advanced breast cancer.

In the stage III TROPiCS02 (NCT03901339) study randomized in patients with HR+/HER2 advanced breast cancer, the treatment of SG compared with TPC showed a numerical difference in OS (13.
9 months versus 12.
3 months) between SG and TPC, but no significant difference
。 The SG group had higher ORR (21% versus 14%) and CBR (34% vs.
22%), median duration of response (DOR) of 7.
4 months versus 5.
6 months, and improved median PFS (5.
5 months vs 4.
0 months)
compared to the TPC group.
The efficacy of SG on residual lesions after neoadjuvant chemotherapy in patients with HER2-negative breast cancer is being studied in phase III (NCT04595565).

In the phase I/II study (NCT01631552), SG showed clinical activity in patients with relapsed or refractory mUC with an ORR of 31%.

NCT03547973 is a phase II trial that confirmed the initial signal
of SG presence in mUC patients.
The ORR was 27%, and the median PFS and OS were 5.
4 and 10.
9 months
, respectively.
In 2021, the FDA accelerated the approval of SG for the treatment of advanced or metastatic UC.

In mUC patients, phase III confirmatory trials of SG versus taxane or vinblastine are ongoing
.

Although SG has shown good anti-tumor effects in clinical trials, it has the same side effects as the toxic payload (SN-38), including myelosuppression and gastrointestinal toxicity
.
At the same time, SG may develop interstitial lung disease (ILD) in normal tissues, which may be due to the non-target effect of
the Trop2 antibody.
More clinical studies are needed to determine whether SG is manageable
in the treatment of cancer side effects other than TNBC.

• Datopotamab deruxtecan(Dato-DXd，DS-1062)

Daiichi Sankyo and AstraZeneca have partnered to develop DS-1062, which is also a Trop2-targeted ADC for the treatment of metastatic breast cancer and metastatic non-small cell lung cancer
.
DS-1062 includes humanized anti-Trop2 IgG1 monoclonal antibody, digestible tetrapeptide GGFG linker, and DNA topoisomerase I inhibitor exatecan derivative (DXd).

This linker can only be cleaved by lysosomal enzymes, which helps to overcome the problem of early drug release to
some extent.
DS-1062 is internalized into Trop2-expressing tumor cells by anti-Trop2 IgG1 monoclonal antibody and transported to lysosomes, where the linker is cleaved, resulting in payload release
.
The cytotoxic mechanism of DS-1062 on cancer cells includes the release of DXd and bystander effects
.
DS-1062 is currently in Phase III clinical trials
.

The safety, tolerability and preliminary efficacy of DS-1062 in patients with advanced solid tumors were evaluated
in a Phase I clinical trial (NCT03401385).
A total of 180 patients with NSCLC received DS-1062 courses of 4 mg/kg (n=50), 6 mg/kg (n=50), or 8 mg/kg (n=80).

The median follow-up period was 11.
4 months
.
blinded independent ORR: 4 mg/kg, 24% (12/50); 6 mg/kg，26%(13/50)； 8 mg/kg，24%(19/80)
。 Emergency adverse events (TEAEs) were observed in 47% of patients ≥ grade 3, with nausea, stomatitis, alopecia, and fatigue being the most common
.
Nineteen (11%) patients had drug-related interstitial lung disease, of which 3 in the 8 mg/kg group had grade
5.

In the same clinical study, 43 patients in the TNBC group received DS-1062
.
DOT was 2.
8 months, ORR was 39%, and disease control was 84%.

TEAE
occurs in 35% of patients ≥ grade 3.
Nausea, stomatitis, hair loss, vomiting, and fatigue are the most common TEAEs
.
No cases of treatment-related interstitial lung disease have been reported
.

In summary, DS-1062 showed antitumor activity and safety
in both non-small cell lung cancer and TNBC at a dose of 4/6/8 mg/kg.
The 6 mg/kg dose was more tolerated and more effective than the 8 mg/kg dose, with a lower
discontinuation rate for adverse events.
Based on the excellent clinical activity of DS-1062 in NCT03401385, Daiichi Sankyo and AstraZeneca conducted a Phase III clinical study of TROPION-Lung01 (NCT04656652) to evaluate the efficacy, safety, and pharmacokinetics
of DS-1062 compared to docetaxel in patients with advanced or metastatic NSCLC with or without operable genomic alterations.

In addition to TROPION-Lung01, DS-1062 has been tested
in several other clinical trials.
TROPION-Lung05 is a phase II study in advanced or metastatic NSCLC in patients who have previously been treated with kinase inhibitors and platinum-based chemotherapy with actionable genomic changes
.
TROPION-Lung04 (NCT04612751) is a Phase 1b trial to evaluate the safety and tolerability
of DS-1062 with or without carbolimab in patients with advanced or metastatic NSCLC.
TROPION-Lung02 (NCT04526691) is a Phase 1b trial to test the safety and therapeutic activity
of DS-1062 in combination with Keytruda in the treatment of advanced or metastatic non-small cell lung cancer 。 Based on the excellent results of TROPION-Lung02, the Phase III clinical trial of TROPION-Lung08 (NCT05215340) was used to compare the efficacy
of DS-1062 in combination with Keytruda or Keytruda alone in the first-line treatment of advanced or metastatic NSCLC without actionable genomic alterations 。 TROPION-Breast01 (NCT05104866) is a Phase III trial to evaluate the safety and efficacy
of DS-1062 in patients with inoperable or metastatic HR+/HER2 breast cancer who have received one or both systemic chemotherapy.
TROPION-Breast02 (NCT05374512) is a Phase III multicenter study to evaluate the efficacy and safety
of DS-1062 compared to selected chemotherapy in patients with locally relapsed, inoperable or metastatic TNBC.

• SKB-264

SKB-264 is a novel Trop2-targeted ADC
developed by Sichuan Kelun Pharmaceutical Co.
, Ltd.
for advanced metastatic solid tumors.
A novel topoisomerase I inhibitor (derived from belotecan) is attached to a human IgG1 mAb via an enzymatifiable linker with a DAR of 7.
4
.
The release of the payload when internalized in SKB-264 is in a way that is relevant to
Trop2 expression.

SKB-264 is currently being tested in a Phase I/II clinical trial (NCT04152499) in patients with locally advanced unresectable/metastatic solid tumors that do not respond
to existing standard therapies.
The study included phase I clinical phase dose escalation and phase II clinical phase dose expansion
.
As of April 2021, 18 patients were assigned to 3 dose groups of 2, 4 and 6 mg/kg, and 55.
6% had received ≥ 4 previous treatment regimens
.
There were 6 cases of TNBC (33.
3%), 5 cases of ovarian cancer (27.
8%), 3 cases of pancreatic cancer (16.
7%), 2 cases of urothelial carcinoma (11.
1%), 1 case of HER2-positive breast cancer, and 1 case
of gastric adenocarcinoma.
All 18 patients experienced TEAE
.
The most common adverse reactions were grade 1~2 nausea (72.
2%) and hair loss (66.
7%)
.
Neutropenia (5/18), leukopenia (4/18), and anemia (3/18) are the most common ≥ grade 3 TEAE
.
Seventeen patients received at least one efficacy evaluation, with an ORR of 41.
2% (7/17) and a DCR of 70.
6% (12/17).

Two of the five TNBC patients received PR (ORR 40%), three of the five ovarian cancer patients received PR (ORR 60%), and one patient with HER2-positive breast cancer received PR (1/1).

One patient with gastric adenocarcinoma achieved PR (1/1).

Another patient with pancreatic cancer was stable after treatment with SKB-264 and had a control time of 30.
3 weeks
.
Preliminary clinical results show that SKB-264 has obvious efficacy in patients with advanced TNBC, and also has a good effect
on other advanced or metastatic solid tumors expressing Trop2.

In April 2022, SKB-264 was approved for a Phase III clinical trial in patients with advanced or metastatic TNBC who have failed at least second-line therapy (CXSL2200208, NCT05347134).

Meanwhile, a phase II clinical trial (NCT05351788) began evaluating the antitumor activity
of SKB264 in combination with KL-A167 in advanced or metastatic non-small cell lung cancer.

• ESG-401

ESG-401 is an ADC
for Trop2 jointly developed by Silgen and Lianning.
It consists of
humanized anti-Trop2 monoclonal antibody conjugated to SN-38.
ESG-401 has a novel, extremely stable and cleavable linker that releases very little free toxins in circulation, enriches in target tissues, and rapidly endocytoses, inhibiting tumor growth
.

ESG-401 has shown an excellent safety profile in preclinical studies with no off-target or off-tumor toxicity
in high-dose, repeated-dose non-human primates.
In addition, ESG-401 showed significant tumor inhibitory activity in a variety of tumor models expressing Trop-2, with a low effective dose and a long
inhibition time on tumor growth.
Based on excellent preclinical data, ESG401 safety and tolerability are being tested in patients with advanced epithelial cancer (NCT04892342, CXSL2101069).

• JS-108

JS-108 (DAC-002) is an ADC for Trop2 jointly developed by Duoxi Biologics and Junshi Biologics for the treatment of Trop2-positive solid tumors
such as TNBC, small cell lung cancer, and pancreatic cancer.
JS-108 consists of
humanized monoclonal antibody against Trop2, 2,3-disubstituted long side chain hydrolysis resistance linker, and tubulysin B analogue Tub196.
The safety, tolerability, PK features and efficacy of JS-108 are currently being evaluated in patients with advanced solid tumors in
the Phase I clinical phase (NCT04601285).

• FDA018

FDA 018 is an anti-tumor ADC for Trop2 developed by Shanghai Fudan Zhangjiang Biologics, and its payload and linker are not disclosed
.
The DMPA has approved the FDA 018 Phase I clinical trial to study its safety, tolerability and pharmacokinetics in patients with advanced solid tumors (CXSL2101031, NCT05174637).

Comparison of novel Trop-2 ADCs in clinical stages

• Payload

Topoisomerase I inhibitors are used as payloads in SG, DS-1062, ESG-401, and SKB-264 to induce DNA damage and ultimately lead to apoptosis
.
The payload of SG and ESG-401 is the active metabolite SN-38
of irinotecan.
The DS-1062 payload is an Exatecan derivative (DXd).

DXd is ten times more toxin-active than SN38, and while DXd may be more effective at interfering with DNA replication and recombination in tumor cells, it also carries a higher risk of
cytotoxicity.
Due to non-target effects, DS-1062 may produce interstitial pneumonia (ILD)
in normal tissues.
SG's payload SN38 is metabolized and cleared by UGT1A1, and UGT1A1 mutates into UGT1A1*28, which will cause SN38 accumulation and toxicity
.

• antibody

The KD value of the SG monoclonal antibody fraction is 0.
3 nM, while the KD value of DS-1062 is 27 nM
.
There is a 90-fold difference
between the two.
Daiichi Sankyo may have chosen a low-affinity antibody to reduce the toxicity
of DS-1062 in normal tissues.
It remains to be seen whether
this balanced design will lead to better clinical outcomes.
There is no antibody information
for SKB-264, ESG-401, JS-108, and FDA 018.

• Connector

In terms of connector selection, SG uses hydrolyzable CL2A type connectors
.
This pH-dependent linker can be hydrolyzed in the tumor's acidic environment, killing surrounding tumor cells
through the bystander effect.
The structure of the SKB-264 connector is highly similar to CL2A, and it is speculated that the SKB264 linker is modified on the basis of CL2A and may have the same mechanism
as CL2A.
Unlike SG, JS-108 uses non-hydrolyzed linkers, which may not produce a bystander effect
.
The linker in DS1062 can only be cleaved by lysosomal enzymes such as cathepsin, which helps to overcome the early release
of the drug to some extent.

• DAR

DAR is an important quality attribute of ADCs and represents the average number of
drugs bound to the antibody.
The DAR values for SG, DS-1062, and SKB-264 are 7.
6, 4, and 7.
4
, respectively.
The DAR and linker of SKB-264 are comparable to SG, but the coupling method adopts methanesulfonic acid reagent and thiol to react through the nucleophilic aromatic substitution mechanism, which will not react with albumin and cause linker-payload shedding, so it is more stable and reduces toxic side effects
.
It will be something to look forward to if SKB-264 can take the place of SG in the same indicators
.
DS-1062 has the lowest DAR value, possibly because Daiichi Sankyo carefully considered Trop2 expression in normal tissues, payload toxicity, and clinical outcomes
.

• Advantages of SG

SG is currently the only ADC approved to treat Trop2-positive tumors, and its success is due in part to its ingenious design
.
First, SG's payload, SN-38, has a shorter half-life than tubulin inhibitors and is not prone to accumulation
in the body.
In addition, because the target number of intracellular tubulin inhibitors far exceeds that of DNA inhibitors, DNA inhibitors have superior cell killing effects
when the same amount of payload penetrates the cells.
At the same time, higher DAR values (DAR=7.
6) allow SG to deliver a higher tolerated dose of cytotoxins than the same systemic administration typically administered alone, which will help minimize toxicity and non-target effects
.
Second, the shorter peg group junction structure overcomes the hydrophobicity of the drug and improves the solubility of the drug in water, which helps fight drug resistance
.
The moderate stability of the SG linker allows for the slow release of SN-38, which may be a key factor
driving SG's tolerance and effectiveness against metastatic TNBCs.
Thirdly, the bystander effect of SG on nearby tumor cells is an important reason for
its excellent antitumor activity.