Ongoing research in cancer therapy has led to the development of antineoplastic agents which target specific components of the cell cycle. In Part II of this series, we discuss agents which target the mitotic mechanism by inhibiting microtubules. Although many of these agents are being shown to have multiple effects, the Vinca alkaloids and the taxanes are known as antimitotic drugs. They are among the most important anticancer agents currently available, and because of their unique mechanisms, can be combined with a wide variety of other antineoplastic agents in a spectrum of diseases. In addition, in part II, we are discussing agents that target DNA and prevent replication and thus cell growth by inhibiting the enzymes which protect DNA during replication, the topoisomerases. These drugs, too, have unique mechanisms of action and have become major components of combination regimens. The topoisomerase I inhibitors are new drugs derived from an older parent drug, and their full possibilities are still being explored.

A B S T R A C T In vivo studies demonstrate that the pancreatic enzymes and the ionic environment in the upper gastrointestinal tract are essential determining factors for transport and absorption of cobalamin in man.Jejunal fluid was aspirated from healthy human volunteers after administration of cyano[57Co]cobalamin preparations. Immunochemical analysis of the aspirates demonstrated that all isotopic vitamin was transferred to a protein that is identical to the gastric intrinsic factor in terms of molecular mass (57,500), ionic nature (mean pI, 5.09), and reactivity with antiintrinsic factor sera. However, in the aspirates from patients with exocrine pancreatic dysfunction the vitamin was found to be coupled >60% to a protein identical to R proteins in terms of molecular mass (125,000), ionic nature (mean pI, 3.51), and reactivity with anti-R protein and anti-intrinsic factor sera. The preferential transfer of cobalamin to R proteins in the patients and to intrinsic factor in healthy subjects was associated, respectively, with low and normal levels ofpancreatic enzymes in the intestine and these in turn were paralleled respectively by impaired and normal ileal absorption of cobalamin. These findings confirm the suggestion that the formation of unabsorbable cobalamin complexes may be the reason of impaired vitamin absorption in exocrine pancreatic insufficiency.This paper was presented in part at the 3rd European Symposium on Vitamin B12 and Intrinsic Factor, March 1979, Zurich, Switzerland.Dr. Marcoullis is a visiting professor of medicine under the auspices of Institut National de la Sante et de la Recherch6 Medicale at the University of Nancy. The permanent address for Dr. Marcoullis and Ms. Jimenez is Department of Medicine, Veterans Administration Hospital, Brooklyn, N. Y. Received for publication 2 November 1979 and in revised form 19 May 1980. 430 Observations made with other selected patients demonstrate: (a) that decreased enzyme activity and nondegradation of R proteins may also be due to nonactivation ofpancreatic zymogens in an acidic pH ofthe intestinal juice and; (b) that in the absence of an acidic environment in gastric juice the vitamin transported to the jejunum couples to intrinsic factor when pancreatic function is normal, and to intrinsic factor and R protein in exocrine pancreatic insufficiency. The observations made with these selected patients may explain why not all patients with exocrine pancreatic insufficiency develop impaired cobalamin absorption, and also why the malabsorption is corrected by the administration of bicarbonate in certain patients. INTRODUCTIONA common feature of the untreated exocrine pancreatic insufficiency (EPI)l is the impaired ileal absorption of cobalamin (Cbl; vitamin B,2) (1-8). The mechanism(s) underlying the pathophysiology of this phenomenon have not been worked out (8-12). The two principal prerequisites for normal Cbl absorption i.e., the complexing of dietary Cbl to the gastric intrinsic factor (IF) (gastric phase of Cbl transport), as well as th...

The vitamin B(12) binders in the pig pyloric mucosa gastric and intestinal juice from the upper gastrointestinal tract were fractionated into only two molecular forms, classified as intrinsic factor and cobalophilin. The unsaturated vitamin B(12)-binding power due to cobalophilin was lower in the intestinal than in the gastric juice. Electrofocusing revealed that intrinsic factor and cobalophilin in the intestinal juice contained more of the ;neutral'-type isoproteins, and the suggestion is made that this is due to enzyme activity. The gastric-juice intrinsic factor contained more acidic isoproteins, which supports the hypothesis that carbohydrate is added on to the polypeptide chain of this protein before it is secreted into gastric juice. The gastric- and intestinal-juice cobalophilins, studied also by electrofocusing, differed from that of pyloric mucosa and they appeared to be of salivary origin. With regard to molecular dimensions there was no significant difference between the intrinsic factors and cobalophilins from all sources studied. All cobalophilins had molecular weights by the formula of Svedberg of approx. 92500, Stokes radii of 4.62nm and sedimentation coefficients of 5.15S. The corresponding values for the intrinsic factors were 63600, 3.57nm and 4.38S. In addition, the intrinsic factors exhibited similar avidities for binding to the solubilized ileal intrinsic-factor receptor. Also the intrinsic factors and cobalophilins, irrespective of their source, bound to the analogous specific xenoantibodies with the same avidity. The present results demonstrate that intrinsic factor remains practically unaltered during its passage through the proximal intestine and render unlikely the speculations made about the presence of an endogenous binder for intrinsic factor as well as the existence of a ;pancreatic intrinsic factor'. In addition, they are compatible with the theory that the interference by undegraded cobalophilin may be the reason for the abnormal vitamin B(12) absorption observed in patients with pancreatic insufficiency.

The purpose of these studies was to determine whether gastric intrinsic factor and the ileal intrinsic factor receptor participate in the process of cobalamin absorption in the dog. Physicochemical analysis of gastrointestinal fluids and mucosal extracts obtained 3-5 h after cyano[57Co]-cobalamin was fed to dogs demonstrated that 1) all cyano-[57Co]cobalamin became bound to proteins during intraluminal transport; and 2) mucosal cyano[57Co]cobalamin in the extract of the ileal mucosa was bound to intrinsic factor, to intrinsic factor coupled to receptor protein, and to proteins with properties similar to R protein and transcobalamin II. A significant fraction of the cyano[57Co]cobalamin in the mucosal extract was membrane bound and, upon solubilization with Triton X-100, was found to contain immunoreactive intrinsic factor that, however, could no longer couple to the isolated receptor. The formation of the complex of cobalamin with intrinsic factor and the receptor protein and the selective accumulation of cobalamin in the ileum indicate that the intrinsic factor-mediated mechanism for absorption of this vitamin is active in the dog.