Peptide, Small Molecule, and Bicycle Drug Conjugates Emerge as Potent Alternatives to ADCs in Solid Tumors

While Antibody Drug Conjugates (ADCs) are a cornerstone in oncology, their therapeutic window in solid tumors is often restricted by systemic toxicities and emerging drug resistance. Challenges include limited tumor penetration and complex manufacturing. There's a critical need for targeted delivery platforms that offer improved efficacy, enhanced tumor selectivity, and differentiated safety profiles to overcome these limitations and expand treatment options for difficult-to-treat cancers.

This narrative review synthesized preclinical and clinical literature on targeted drug conjugates, specifically examining antibody fragment-drug conjugates, small molecule-drug conjugates (SMDCs), peptide drug conjugates (PDCs), and Bicycle drug conjugate (BDC) molecules. Literature searches were conducted on PubMed and ClinicalTrials.gov, without specific term or date constraints, to discuss their development, shortcomings, and refinement avenues compared to traditional ADCs.

The review highlights that novel SMDCs, PDCs, and BDC molecules are poised to overcome current challenges associated with cytotoxin-containing ADCs. These next-generation conjugates offer several advantages, including improved tissue penetration due to their smaller size, enhanced synthetic accessibility, and the potential for differentiated safety profiles. PDCs, for instance, leverage tumor-homing peptides for precise delivery, often exhibiting better tumor-to-blood ratios. SMDCs utilize small molecule ligands to target specific receptors, allowing for rapid clearance and reduced systemic exposure. BDC molecules, with their unique bicyclic peptide structure, combine high affinity with rapid tumor penetration and renal clearance. Emerging preclinical and clinical data for these platforms are highly promising, with BDC molecules already under pivotal randomized phase II clinical investigation. The authors anticipate these novel treatments will translate into regulatory approvals, offering significant advancements for patients.

PDCs, SMDCs, and BDCs are expected to circumvent current challenges with cytotoxin-containing ADCs, offering differentiated treatment options for difficult-to-treat solid tumors.