Targeted protein degradation (TPD) is an emerging therapeutic modality and has attracted great attention from academia and industry^1,2. The prototypical TPD agents, molecular glues (MGs) and proteolysis targeting chimeras (PROTACs), can lead to temporal proteasomal degradation of the protein-of-interest (POI). PROTACs are small heterobifunctional molecules integrating an E3-ligase binder and a POI binding moiety through a synthetic linker construct. The PROTACs technology has been applied to degrade numerous pathological proteins and a rich pipeline is currently progressing into preclinical and early clinical trials^3,4,5. However, overcoming PK/PD issues towards clinical compounds is demanding due to the intrinsically high molecular weight and related physicochemical properties⁶. On the other hand, MGs are small molecules with beneficial ‘drug-like’ physicochemical properties binding to an E3 ligase, and, similarly to PROTACs, leading to neosubstrate proteasomal degradation. Their mechanism of action is however less predictable; their often hydrophobic surface-exposed portions of the MGs seem to change the hydrophobic surface area of the E3 ligase and thereby leading to neosubstrate ubiquitination and degradation^7,8. MGs have already proven their validity as marketed drugs, as there are several approved drugs or clinical compounds working by an MG mechanism (Fig. 1A), for example, the IKZF1/3 degrader thalidomide and its analogs pomalidomide and lenalidomide⁸, and the RBM39 degrader indisulam⁹. Thalidomide analogs induce selective ubiquitination and degradation of two lymphoid transcription factors, IKZF1 and IKZF3, by the CRBN-CRL4 ubiquitin ligase¹⁰. Additionally, CSNK1A1 (CK1α) was recently discovered as a lenalidomide-specific neo-substrate¹¹. Interestingly, modification of pomalidomide or lenalidomide can have a profound impact on the degradation potency and degradation profiles. For example, CC-220 (Fig. 1A) showed 10-fold more potency in the cells than lenalidomide, and CC-885 (Fig. 1A) was found to induce degradation of the substrate GSPT1^12,13. Both MGs and PROTACs are emerging drug modalities providing interesting features over classical pharmacology-driven drugs by their ability to drive the destruction of proteins that have multiple functions, thereby potentially overcoming resistance mechanisms and providing new pharmacology. While PROTACs can be developed highly rationally, MGs are discovered rather serendipitously requiring synthesis and testing of large series of compounds^14,15. Additionally, the discovery of MGs and PROTACs is done in a sequential, often mmol scale synthesis which is time-consuming and expensive.

In this work, to address current shortcomings in MGs discovery, we use the direct-to-biology (D2B) approach and combined the automated, high throughput miniaturized synthesis with cell-based phenotypic screening (Fig. 1B). The I.DOT (Immediate Drop on Demand Technology, a pressure-based nano dispensing technology) is employed to accelerate the synthesis of diverse MGs libraries on nano scale^{16,17,18,19,20,21}. In a subsequent cell-based phenotypic screening cascade, the compounds are tested in the thalidomide and analog sensitive MM.1S multiple myeloma cell line which reportedly is used for MGs screening²². In this D2B screening platform, the crude compounds are directly screened on cells without further chromatographic purification or clean up. Then, the 19 best compounds are selected for re-synthesis on mmol scale followed by purification and fully characterized.