Abstract LB003: Fragment based discovery of MRTX9768, a synthetic lethal-based inhibitor designed to bind the PRMT5-MTA complex and selectively target MTAP/CDKN2A-deleted tumors

The MTAP gene is proximal to and co-deleted in nearly all CDKN2A-deleted cancers. This genetic alteration is present in an estimated 9% of all cancers and is especially prevalent in cancers with high unmet medical need (e.g. mesothelioma (32%), pancreatic (22%), lung squamous (20%)). Multiple independent research teams have demonstrated that tumor cell lines with homozygous MTAP deletions are hypersensitive to shRNA-mediated knock down of PRMT5. MTAP is required for the methionine salvage pathway and MTAP-del cells accumulate MTA, an inhibitory co-factor which competes for binding to the co-factor binding site of PRMT5 with the activating co-factor SAM. PRMT5 is a methyltransferase that adds symmetric dimethylarginine (SDMA) modification to proteins and is essential for mammalian cell survival. A small molecule that selectively binds and stabilizes the catalytically inactive PRMT5•MTA complex may represent a synthetic lethal-based precision medicine for the treatment of MTAP/CDKN2A—del tumors. Notably, 1st generation PRMT5 small molecule inhibitors do not target MTA-complexed PRMT5 and do not exhibit selective inhibition of MTAP-del cancer cells. Here we report a new series of compounds discovered via a fragment-based approach that selectively bind to the PRMT5•MTA complex. A fragment hit was identified in an SPR binding assay with PRMT5•MTA (KD 18 μM). The binding mode was determined by X-ray crystallography and revealed that the fragment makes productive interactions with K333, F327, E435, E444, E435, and W579 of PRMT5 as well as with the co-liganded MTA. Fragment growing aided by structure-based design identified a key interaction with the L312 backbone N-H that enhances binding to PRMT5•MTA (MRTX4646, SPR KD 57 nM). Further exploration highlighted an interaction with the F580 backbone N-H as important for cellular activity and selectivity. This interaction with F580 was illustrated by MRTX7512 which exhibits an IC50 value of 633 nM for inhibition of SDMA in engineered HCT116 MTAP-del cells and demonstrates 15-fold selectivity compared with HCT116 MTAP-WT cells (IC50 9763 nM). Further optimization of cellular potency and pharmacokinetic properties identified MRTX9768, a potent inhibitor of SDMA and cell proliferation in HCT116 MTAP-del cells (SDMA IC50 3 nM; prolif. IC50 11 nM) with marked selectivity over HCT116 MTAP-WT cells (SDMA IC50 544 nM; prolif. IC50 861 nM). In xenograft studies, oral administration of MRTX9768 demonstrates dose-dependent inhibition of SDMA in MTAP-del tumors, with less SDMA modulation observed in bone marrow. In summary, we have used a fragment-based approach to discover a new class of orally active PRMT5•MTA inhibitors that demonstrate selective antitumor activity in MTAP-del tumor cells while sparing MTAP-WT cells.

Citation Format: Christopher R. Smith, Svitlana Kulyk, J. D. Lawson, Lars D. Engstrom, Ruth Aranda, David M. Briere, Robin Gunn, Krystal Moya, Lisa Rahbaek, Laura Waters, Anthony Ivetac, James G. Christensen, Peter Olson, Matthew A. Marx. Fragment based discovery of MRTX9768, a synthetic lethal-based inhibitor designed to bind the PRMT5-MTA complex and selectively target MTAP/CDKN2A-deleted tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB003.

Intrinsically disordered protein

Proteins can exist in a trinity of structures: the ordered state, the molten globule, and the random coil. The five following examples suggest that native protein structure can correspond to any of the three states (not just the ordered state) and that protein function can arise from any of the three states and their transitions. (1) In a process that likely mimics infection, fd phage converts from the ordered into the disordered molten globular state. (2) Nucleosome hyperacetylation is crucial to DNA replication and transcription; this chemical modification greatly increases the net negative charge of the nucleosome core particle. We propose that the increased charge imbalance promotes its conversion to a much less rigid form. (3) Clusterin contains an ordered domain and also a native molten globular region. The molten globular domain likely functions as a proteinaceous detergent for cell remodeling and removal of apoptotic debris. (4) In a critical signaling event, a helix in calcineurin becomes bound and surrounded by calmodulin, thereby turning on calcineurin's serine/threonine phosphatase activity. Locating the calcineurin helix within a region of disorder is essential for enabling calmodulin to surround its target upon binding. (5) Calsequestrin regulates calcium levels in the sarcoplasmic reticulum by binding approximately 50 ions/molecule. Disordered polyanion tails at the carboxy terminus bind many of these calcium ions, perhaps without adopting a unique structure. In addition to these examples, we will discuss 16 more proteins with native disorder. These disordered regions include molecular recognition domains, protein folding inhibitors, flexible linkers, entropic springs, entropic clocks, and entropic bristles. Motivated by such examples of intrinsic disorder, we are studying the relationships between amino acid sequence and order/disorder, and from this information we are predicting intrinsic order/disorder from amino acid sequence. The sequence-structure relationships indicate that disorder is an encoded property, and the predictions strongly suggest that proteins in nature are much richer in intrinsic disorder than are those in the Protein Data Bank. Recent predictions on 29 genomes indicate that proteins from eucaryotes apparently have more intrinsic disorder than those from either bacteria or archaea, with typically > 30% of eucaryotic proteins having disordered regions of length > or = 50 consecutive residues.

Isolation of chloroform-resistant mutants of filamentous phage: localization in models of phage structure

Interaction of fd or M13 filamentous phage with a chloroform/water interface induces morphological change, contracting the filaments sequentially into shortened rods (I-forms), and then into spheroidal particles (S-forms). To further investigate this phage contraction, 34 and 26 chloroform-resistant isolates of fd and M13, respectively, were selected after chloroform treatment of wild-type phages at pH 8. 2 and 4 degrees C. DNA sequencing of gene VIII of the 34 fd isolates revealed five different mutants: these were D5H, M28L, V31L, I37T, and S50T. All 26 M13 isolates were I37T. These mutants exhibited variable sensitivity to chloroform, but all contracted much more slowly than wild-type phage during treatment at 4 degrees C. They all contracted like wild-type phage at 37 degrees C. Site-directed mutagenesis showed that the indicated single mutations carried the chloroform resistance. In structural models of the phage, the D5H locus is on the outside and the S50T locus is on the inside. The M28L and I37T loci are buried in a mostly hydrophobic region in the middle. Although these four mutants are spread out radially, they are localized in the axial direction into a thin disk in the model. The last mutant locus, V31L, is out of this disk, but this locus is proximal to the M28L and I37T loci and also in contact with the surface via a deep hydrophobic hole or depression. These five mutants, their locations, and their variable affects on contraction suggest that chloroform-induced contraction involves a specific mechanism rather than a generalized solvent-induced denaturation and that the critical structural changes occur in a localized level in the phage. These results add weight to suggestions that the sequential contraction of filaments-->I-forms-->S-forms mimic corresponding steps in phage penetration, and, in the reverse order, for phage assembly.

Guidelines for managing low back pain: a policy checklist

Back injuries devour one-third of health care benefits and workers' compensation, yet corporate strategies vary widely in their approach to back pain prevention, treatment and rehabilitation. These recommendations provide a benchmark for evaluation policies on low back pain. They emphasize a sports medicine approach, focused on returning workers to activity as soon as possible.

Homologous veins as an arterial substitute: long-term results

From 1968 through 1982, 129 homologous vein grafts were used in 91 patients in the following positions: 75 in the femoropopliteal, tibial, or peroneal artery; 38 in the aortocoronary artery; 13 in the aortopulmonary artery; and one each in the atrioventricular fistula, carotid-subclavian artery, and brachial-radial artery. In the lower extremity patency ranged from 0 to 121 months (mean 22.4 +/- 4.4 months). All grafts were performed for limb salvage, and 75% of the patients had undergone previous operations. Cumulative patency by the life-table method showed that while 50% of grafts occluded by 1 year, 60% of the remaining grafts continued to be functional for more than 5 years. Fifty percent of the aortocoronary bypass grafts studied were occluded at 1 year. Eight of the 13 systemic pulmonary artery shunts were patent at time of death, revision, or total correction. Multiple revisions and thrombectomies are required to maintain patency of homograft veins. The outcome is variable and unpredictable. The inconsistency is due to the antigenicity of the graft. If one is committed to the necessary efforts required to ensure long-term patency, the homologous saphenous vein is a suitable substitute when autogenous tissue is unavailable.

Median arcuate ligament syndrome with severe two-vessel involvement

Intestinal angina is an unusual condition caused by decreased blood supply to the abdominal viscera. It has been hypothesized that at least two of the three vessels supplying the viscera need to be compromised to cause ischemia. On the other hand, compression of the celiac axis by the medium arcuate ligament, causing symptoms, has been reported. We described a severely symptomatic patient in whom this ligament completely occluded the celiac axis and severely narrowed the superior mesenteric artery. The condition was cured by division of the ligament.

Three simple solutions for renal flush and preservation

The harvesting of kidneys at small hospitals where standard flush solutions are not available has led to the use of several flush solutions that can be made from readily available materials. Mongrel dogs (15) had bilateral nephrectomy after receiving intravenously a mixture of Ringer's lactate, furosemide (Lasix), mannitol, and chlorpromazine (Thorazine). One kidney was perfused with iced Collins' solution and stored for 24 hours, submerged in the perfusate at 4 C. Five dogs each had the contralateral kidney perfused with a solution of either (1) Ringer's lactate, salt-poor albumin, and heparin; (2) 6% dextran 70 and Fenwall buffered heparin solution; or (3) Ringer's lactate, mannitol, and heparin, and were stored in their respective perfusate. After 24 hours' storage the kidneys were transplanted into the neck, and creatinine clearance (Crcl) was determined on days 1, 2, 3, 5, and 7. The dogs were sacrificed on day 7 and biopsy specimens were obtained from each kidney for pathologic examination. The kidneys perfused with Collins' solution had immediate function after transplantation with an average Crcl of 10.5 at 25 hours and 20.2 at seven days. The kidneys perfused with Ringer's lactate and albumin had minimal function immediately after transplantation and were anuric within 24 hours. The kidneys perfused with dextran solution and Ringer's lactate and mannitol solution had immediate function but output remained low, and by seven days the average Crcl was only 3.0 and 4.5 respectively. All solutions except the Ringer's lactate and albumin were hyperosmolar, thus helping to prevent the "no reflow" phenomenon. These results indicate that the solutions tested do not result in the adequate preservation to warrant clinical use.

Resection of abdominal aortic aneurysms in the over-80 age group

We reviewed the risk of resecting an abdominal aortic aneurysm in 38 patients aged 80 years or older. The operative mortality for an elective aneurysm was 5.2%, for symptomatic but nonruptured aneurysm 26.6%, and for ruptured aneurysm 66.7%. The preoperative risk factors were essentially the same for the elective and ruptured group. Several of the patients with ruptured aneurysm had been followed up with their aneurysm for several years by nonsurgeons. To better understand why these patients were not being referred for elective repair, 100 nonsurgeons were sent a questionnaire concerning abdominal aortic aneurysms in octogenarians. The results showed that 81% overestimated the elective operative mortality and as a result did not recommend elective resection.

Revascularization of the poorly functioning kidney

The effect of renal vascularization on renal function in kidneys with poor preoperative excretory function is summarized in this report. Twenty-five patients with kidneys having preoperative creatinine clearance of less than 30 ml/min, as determined during split renal function studies (SRFS), which were revascularized for treatment of secondary renovascular hypertension and then were reevaluated by repeat SRFS form the basis of this report. There were 13 male and 12 female patients. The type of renal artery lesion was atherosclerotic in 21 patients and fibromuscular dysplastic in four patients. Eight individuals had total renal artery occlusion. Significant contralateral renal artery disease was present in 15 patients (60%). Preoperative creatinine clearances in the affected kidneys ranged from 0.27 ml/min (mean, 16 +/- 9 ml/min). after operation, creatinine clearances ranged from 0 to 72 ml/min (mean, 32 +/- 16 ml/mn). Fifteen of the 16 kidneys with preoperative creatinine clearances less than 20 ml/min had improvement in renal function following revascularization (P less than 0.01). Improvement (60%) or cure (36%) in hypertension followed revascularization in 24 of the 25 patients. The most dependable predictor of successful management of both hypertension and retrieval of renal function in these patients was the arteriographic demonstration of a patent distal vessel without evidence of severe intrarenal stenoses. These results support an aggressive attitude toward the use of revascularization in the operative treatment of such patients with renovascular hypertension, even when the residual excretory function is minimal or absent.

Hypertension secondary to complete occlusion of the renal artery

In summary, we feel the results of this study support an aggressive approach toward the diagnostic evaluation and operative management of patients with renovascular hypertension secondary to complete occlusion of the renal artery. Further, the frequent presence of contralateral disease and the progressive nature of renal artery occlusions represent a significant threat to renal function. Since renal function is frequently compromised but potentially retrievable by revascularization, nephrectomy should be employed only when hypertension is difficult to control, revascularization is impossible, and excretory function in the affected kidney is minimal.