## 5 Therapeutic Delivery  
Antibodies have become powerful macromolecular therapeutics against a wide range of diseases, including notably cancer [88]. Several of them are amongst the best-selling biologists [89]. Two developments have created an additional momentum: The emergence to the market of antibody-drug conjugates for direct tumor targeting [90], and of immune checkpoint inhibitors for antitumor immunotherapy [91; 92]. In a recent prioritization of cancer antigens by the National Cancer Institute at Rockville (USA), 4 out of 75 were GSLs (GD2, GD3, fucosyl-GM1, and GM3), with GD2 arriving already at the 12 th position [93]. Yet, GSLs are only rarely exploited for tumor targeting. Generating high affinity binding ligands against GSLs is a notoriously difficult task due to the high structural flexibility of these molecules. An exception is the anti-GD2 antibody dinuuximab, which entered the market for the treatment of high-risk neuroblastoma patients [94]. The GL-Lect cargo protein STxB, a natural GSL ligand, was therefore developed as an alternative scaffold for antibodies to exploit GSLs as therapeutic targets. In an initial approach, it was attempted to target the natural STxB receptor, Gb3, which is overexpressed by certain tumors (e.g., breast cancer [95], colorectal carcinoma [96; 97; 98], gastric adenocarcinoma [99], pancreatic cancer [100], and lymphomas [101]; reviewed in Reference [102]). In animal models it was shown that STxB accumulated on Gb3 positive tumors in vivo [97; 103; 104], and that the poorly immunogenic protein could target these tumors even after repeated injections [97]. Synthetic chemistry approaches were used to generate autoimmune linker arm-based conjugates between STxB and the following cytotoxic molecules: the topoisomerase I inhibitor SN38 (Reference [105]), the pro-apoptotic benzodiazep-4864 (Reference [106]), the intercalating drug doxorubicin and the highly cytotoxic tubulin polymerization inhibitor monomethyl auristatin F (MMAF) [107; 108]. On cells in culture, these conjugates had excellent specificity for Gb3 expressing tumors, and for STxB-MMAF activities in the low nanomolar range were measured [107; 108]. In mouse tumor models, these conjugates were unfortunately not successful, most likely due the high Gb3 expression levels on kidney. Very recently, a proprietary methodology has been designed to select for STxB variants that recognize GSLs of choice (patent filing WO2018192719A1). It might thereby become possible to evolve STxB into an alternative molecular scaffold for the targeting of truly tumor specific GSLs or cocktails thereof. The therapeutic success of inhibitory antibodies against the immune checkpoint molecules PD-1/PD-L1 or CTLA-4 has boosted the general interest in antitumor immunotherapy. Yet, only a fraction of patients (below 30%) respond to checkpoint inhibitory antibodies, and those who respond have tumors that are infiltrated by anti-tumor T cells [109]. It is therefore assumed that vaccines that induce such anti-tumor T cells would synergize with checkpoint inhibitor treatments. The GL-Lect cargo protein STxB is one vaccine candidate. Its cellular receptor, the GSL Gb3, is expressed on human and mouse dendritic cells [110; 111; 112]. When chemically coupled to antigenic peptides or proteins, STxB was shown to have a protective effect in mouse models of viral infection [113] or tumor development [114; 115; 116; 117; 118; 119; 120; 121; 122]. Several aspects of STxB as a vector for immunotherapy are of particular interest. First, it was shown early on that STxB synergized with antibodies against the immune checkpoint molecule PD-L1 in the protection of mice against head-and-neck cancers [117]. Second, STxB coupled to the epitopes of the E7 protein from human papilloma virus 16 induced humoral IgA and cellular CD8+ immune responses in the mucosa of the respiratory tract specifically only when given via the mucosal route of vaccination [116; 117; 118; 119].