Neoantigen-based personalized cancer vaccines: the emergence of precision cancer immunotherapy.

INTRODUCTION: The field of cancer therapy has undergone a major transformation in less than a decade due to the introduction of checkpoint inhibitors, the advent of next generation sequencing and the discovery of neoantigens. The key observation that the breadth of each patient's immune response to the unique mutations or neoantigens present in their tumor is directly related to their survival has led oncologists to focus on driving immune responses to neoantigens through vaccination. Oncology has entered the era of precision immunotherapy, and cancer vaccine development is undergoing a paradigm shift. AREAS COVERED: Neoantigens are short peptide sequences found in tumors, but not noncancerous tissues, the vast majority of which are unique to each patient. In addition to providing a description of the distinguishing features of neoantigen discovery platforms, this review will address cross-cutting personalized cancer vaccine design themes and developmental stumbling blocks. EXPERT OPINION: Immunoinformatic pipelines that can rapidly scan cancer genomes and identify 'the best' neoantigens are in high demand. Despite the need for such tools, immunoinformatic methods for identifying neoepitopes in cancer genomes are diverse and have not been well-validated. Validation of 'personalized vaccine design pipelines' will bring about a revolution in neoantigen-based vaccine design and delivery.

Site-specific Labeling of a Protein Lysine Residue By Novel Kinetic Labeling Combinatorial Libraries.

The first example of a kinetic labeling library designed to enable the discovery of affinity labels is presented. Each library component (1) consists of a variable peptidyl component linked to a biotinyl moiety by a 4-mercaptobenzoyl linker in thioester format. We demonstrate that an affinity label can be uncovered by measuring reaction rates between library pools and the protein target, human serum albumin (HSA) and identifying significant outliers. By choosing peptide functionality compatible with a potentially reactive thioester labeling entity, libraries can be screened in pools. It is noteworthy that a limited subset of amino acids (R, S, E, F, Y, l, M, W, and Q) that compose the affinity moiety is sufficient to produce rate variances that guide the discovery process. After two rounds of deconvolution, J-FLYEE-NH2 (7-E) emerges as a bona fide affinity label of HSA. Unlike known affinity labels, the affinity moiety is not retained in the protein product, but is extruded upon acylation of the protein. This feature affords a method of introducing various payloads, without extraneous elements, onto protein frameworks.

Preexposure prophylaxis with albumin-conjugated C34 peptide HIV-1 fusion inhibitor in SCID-hu Thy/Liv mice.

PC-1505 is a C34 peptide derived from the heptad repeat 2 region of HIV-1 gp41 conjugated to human serum albumin for sustained in vivo activity. One single preexposure dose of PC-1505 reduced viral RNA in HIV-1-infected SCID-hu Thy/Liv mice by 3.3 log10 and protected T cells from virus-mediated depletion. In contrast, a single preexposure dose of Truvada reduced viral RNA by only 0.8 log10 and was substantially less effective in preventing T cell depletion.

Permanent inhibition of viral entry by covalent entrapment of HIV gp41 on the virus surface.

HIV entry occurs by concerted conformational changes in the envelope protein complex on the surface of the virus. This complex is made up of a trimer of heterodimers of two subunits: surface subunit, gp120, and transmembrane subunit, gp41. Conformational changes in the envelope complex allow gp41 to mediate membrane fusion leading to exposure of two gp41 regions: N-heptad repeat (NHR) and C-heptad repeat (CHR). Peptides from the NHR or the CHR have been found to inhibit HIV entry. Herein we show that we can covalently inhibit HIV viral entry by permanently trapping the gp41 intermediate on the virus surface using a covalently reactive group on inhibitory peptides. This is evidence showing that vulnerable conformational intermediates exist transiently during HIV viral entry, and the details presented herein will facilitate development of envelope as a target for therapeutics and potential chemopreventive agents that could disable the virus before contact with the host cell.

Albumin-conjugated C34 peptide HIV-1 fusion inhibitor: equipotent to C34 and T-20 in vitro with sustained activity in SCID-hu Thy/Liv mice.

Entry inhibitors of human immunodeficiency virus, type 1 (HIV-1) have been the focus of much recent research. C34, a potent fusion inhibitor derived from the HR2 region of gp41, was engineered into a 1:1 human serum albumin conjugate through stable covalent attachment of a maleimido-C34 analog onto cysteine 34 of albumin. This bioconjugate, PC-1505, was designed to require less frequent dosing and less peptide than T-20 and was assessed for its antifusogenic activity both in vitro and in vivo in the SCID-hu Thy/Liv mouse model. PC-1505 was essentially equipotent to the original C34 peptide and to T-20 in vitro. In HIV-1-infected SCID-hu Thy/Liv mice, T-20 lost activity with infrequent dosing, whereas the antiviral potency of PC-1505 was sustained, and PC-1505 was active against T-20-resistant ("DIV") virus with a G36D substitution in gp41. The in vivo results are the direct result of a significantly improved pharmacokinetic profile for the C34 peptide following albumin conjugation. Contrary to previous reports that the gp41 NHR trimer is poorly accessible to C34 fused to protein cargoes of increasing size (Hamburger, A. E., Kim, S., Welch, B. D., and Kay, M. S. (2005) J. Biol. Chem. 280, 12567-12572), these results are the first demonstration of the capacity for a large, endogenous serum protein to gain unobstructed access to the transient gp41 intermediates that exist during the HIV fusion process, and it supports further development of albumin conjugation as a promising approach to inhibit HIV-1 entry.

A covalent inhibitor targeting an intermediate conformation of the fusogenic subunit of the HIV-1 envelope complex.

Peptide inhibitors corresponding to sequences in the six helix bundle structure of the fusogenic portion (gp41) of the HIV envelope glycoprotein have been successfully implemented in preventing HIV entry. These peptides bind to regions in HIV gp41 transiently exposed during the fusion reaction. In an effort to improve upon these entry inhibitors, we have successfully designed and tested peptide analogs composed of chemical spacers and reactive moieties positioned strategically to facilitate covalent attachment. Using a temperature-arrested state prime wash in vitro assay we show evidence for the trapping of a pre-six helix bundle fusion intermediate by a covalent reaction with the specific anti-HIV-1 peptide. This is the first demonstration of the trapping of an intermediate conformation of a viral envelope glycoprotein during the fusion process that occurs in live cells. The permanent specific attachment of the covalent inhibitor is projected to improve the pharmacokinetics of administration in vivo and thereby improve the long-term sustainability of peptide entry inhibitor therapy and help to expand its applicability beyond salvage therapy.

Synthesis and evaluation of insulin-human serum albumin conjugates.

A series of human insulin maleimido derivatives with short and long linkers was synthesized by exploiting the variations in the pK(a) values and environment of the three amino groups present in the protein. The syntheses were accomplished in organic solvent because of maleimide's instability in basic aqueous media. The derivatives thus obtained were conjugated to the free thiol on Cys34 of human serum albumin (HSA) and purified. A structure-activity relationship based on in vitro receptor binding and activation results for this series of insulin-HSA conjugates showed that the best compounds were attached at the B1 position of insulin with either short or long linkers. Two conjugates were administered subcutaneously to streptozotocin-induced diabetic rats and found to possess blood glucose normalizing activity up to 8 h post-administration. The return to diabetic plasma glucose levels was not observed within the time frame of the experiment (48 h). In comparison, the insulin-treated group's normalization activity lasted 2 h and returned to a diabetic level at 8 h. The onset of the conjugate activities were delayed by 1 h when compared to the activity of human insulin. The study results led to the identification of CJC-1575 as a potent and long lasting human insulin analogue.

Human growth hormone-releasing factor (hGRF)1-29-albumin bioconjugates activate the GRF receptor on the anterior pituitary in rats: identification of CJC-1295 as a long-lasting GRF analog.

In vivo bioconjugation to the free thiol on Cys34 of serum albumin by a strategically placed reactive group on a bioactive peptide is a useful tool to extend plasma half-life. Three maleimido derivates of human GH-releasing factor (hGRF)(1-29) were synthesized and bioconjugated to human serum albumin ex vivo. All three human serum albumin conjugates showed enhanced in vitro stability against dipeptidylpeptidase-IV and were bioactive in a GH secretion assay in cultured rat anterior pituitary cells. When the maleimido derivatives were individually administered sc to normal male Sprague Dawley rats, an acute secretion of GH was measured in plasma. The best compound, CJC-1295, showed a 4-fold increase in GH area under the curve over a 2-h period compared with hGRF(1-29). CJC-1295, a tetrasubstituted form of hGRF(1-29) with an added N epsilon-3-maleimidopropionamide derivative of lysine at the C terminus, was selected for further pharmacokinetic evaluation, where it was found to be present in plasma beyond 72 h. A Western blot analysis of the plasma of a rat injected with CJC-1295 showed the presence of a CJC-1295 immunoreactive species on the band corresponding to serum albumin, appearing after 15 min and remaining in circulation beyond 24 h. These results led to the identification of CJC-1295 as a stable and active hGRF(1-29) analog with an extended plasma half-life.

Identification of CJC-1131-albumin bioconjugate as a stable and bioactive GLP-1(7-36) analog.

A series of analogs of GLP-1(7-36) amide containing a Nepsilon-(2-[2-[2-(3-maleimidopropylamido)ethoxy]ethoxy]acetyl)lysine has been synthesized and the resulting derivatives were bioconjugated to Cys34 of human serum albumin (HSA). The GLP-1-HSA bioconjugates were analyzed in vitro to assess the stabilizing effect of bioconjugation in the presence of DPP-IV as well as GLP-1 receptor binding and activation. Compound 9 (CJC-1131) having the point of attachment to albumin at the C-terminal of GLP-1 and a D-alanine substitution at position 8 was identified as having the best combination of stability and bioactivity.

Kringle 5 peptide-albumin conjugates with anti-migratory activity.

Three peptide fragments of the kringle 5 region of plasminogen and their respective N- and C-terminus maleimido derivatives conjugated to Cys34 of human serum albumin were evaluated in vitro using a human umbilical vein endothelial cell (HUVEC) migration assay and a human plasma stability assay. The N-terminus maleimido derivative of the 64 to 74 segment of kringle 5 conjugated to human serum albumin possessed remarkable anti-migratory activity.

Synthesis and in vitro analysis of atrial natriuretic peptide-albumin conjugates.

Atrial natriuretic peptide (ANP) is a clinically useful anti-hypertensive hormone. Maleimide derivatives of ANP have been synthesized and conjugated to cysteine-34 of human serum albumin. The conjugates were analyzed to assess their stability, receptor binding affinity and ability to stimulate guanylyl-cyclase activity in rat lung fibroblasts.

Development and characterization of a glucagon-like peptide 1-albumin conjugate: the ability to activate the glucagon-like peptide 1 receptor in vivo.

The rapid degradation of native glucagon-like peptide 1 (GLP-1) by dipeptidyl peptidase-IV (DPP-IV) has fostered new approaches for generation of degradation-resistant GLP-1 analogues. We examined the biological activity of CJC-1131, a DPP-IV-resistant drug affinity complex (DAC) GLP-1 compound that conjugates to albumin in vivo. The CJC-1131 albumin conjugate bound to the GLP-1 receptor (GLP-1R) and activated cAMP formation in heterologous fibroblasts expressing a GLP-1R. CJC-1131 lowered glucose in wild-type mice, but not in GLP-1R-/- mice. Basal glucose and glycemic excursion following glucose challenge remained significantly reduced 10-12 h following a single injection of CJC-1131. Twice daily administration of CJC-1131 to db/db mice significantly reduced glycemic excursion following oral and IP glucose challenge (P < 0.01 to 0.05) but did not significantly lower body weight during the 4-week study period. Levels of random fed glucose were significantly lower in CJC-1131-treated +/+ and db/db mice and remained significantly lower even 1 week following discontinuation of CJC-1131 administration. CJC-1131 increased levels of pancreatic proinsulin mRNA transcripts, percent islet area, and the number of bromodeoxyuridine-positive islet cells. These findings demonstrate that an albumin-conjugated DAC:GLP-1 mimics the action of native GLP-1 and represents a new approach for prolonged activation of GLP-1R signaling.