Antibody−drug conjugates (ADCs) have recently demonstrated impressive successes in targeted drug delivery. Ultrasmall (<10 nm) nanoparticle−drug conjugates (NDCs) share many similarities with ADCs, while their unique physicochemical properties can be further molecularly engineered to overcome the limitations of ADCs presented by tumor heterogeneity. Key challenges in NDC development include linkage chemistry design between nanoparticle carriers and cytotoxic drugs, as well as meeting the stringent criteria for manufacturing controls, stability, and drug release to enable successful clinical translation. Here, we report a robust chemical approach to covalently link both chemotherapeutic drugs and targeting moieties to a poly(ethylene glycol) (PEG)-coated (PEGylated) ultrasmall silica nanoparticle platform via precisely tailoring the particle surface chemistry. This approach employs the interstitial space between PEG chains on the particle surface to load drugs, enabling the significantly enhanced drug loading capacity as compared to ADCs while the favorable biodistribution and pharmacokinetics profiles are maintained. To achieve both high plasma stability and effective drug release in cancer, cyclopentadiene silane molecules are first inserted into the PEG layer of the particles and condensed with silanol groups on the silica core surface. Via the Diels−Alder reaction, the cyclopentadiene groups are then functionalized with groups enabling click chemistry, and cytotoxic payloads are finally clicked onto the particles via cleavable linkers for drug release within the cancer tissue. The targeted NDC resulting from the systematic screening strategy described here has recently advanced to a phase 1/2 human clinical trial.

To address the key challenges in the development of next-generation drug delivery systems (DDS) with desired physicochemical properties to overcome limitations regarding safety, in vivo efficacy, and solid tumor penetration, an ultrasmall folate receptor alpha (FRα) targeted silica nanoparticle (C’Dot) drug conjugate (CDC; or folic acid CDC) was developed. A broad array of methods was employed to screen a panel of CDCs and identify a lead folic acid CDC for clinical development. These included comparing the performance against antibody–drug conjugates (ADCs) in three-dimensional tumor spheroid penetration ability, assessing in vitro/ex vivo cytotoxic efficacy, as well as in vivo therapeutic outcome in multiple cell-line-derived and patient-derived xenograft models. An ultrasmall folic acid CDC, EC112002, was identified as the lead candidate out of >500 folic acid CDC formulations evaluated. Systematic studies demonstrated that the lead formulation, EC112002, exhibited highly specific FRα targeting, multivalent binding properties that would mediate the ability to outcompete endogenous folate in vivo, enzymatic responsive payload cleavage, stability in human plasma, rapid in vivo clearance, and minimal normal organ retention organ distribution in non-tumor-bearing mice. When compared with an anti-FRα-DM4 ADC, EC112002 demonstrated deeper penetration into 3D cell-line-derived tumor spheroids and superior specific cytotoxicity in a panel of 3D patient-derived tumor spheroids, as well as enhanced efficacy in cell-line-derived and patient-derived in vivo tumor xenograft models expressing a range of low to high levels of FRα. With the growing interest in developing clinically translatable, safe, and efficacious DDSs, EC112002 has the potential to address some of the critical limitations of the current systemic drug delivery for cancer management.