(Bis-1,7-S-Acetamidomethyl-L-Cysteine)salmon Calcitonin

This invention relates to partially protected calcitonin analogs having biological activity and to processes for preparing such peptides and calcitonins.

All known natural calcitonin peptides contain an amino acid sequence of 32 amino acids, and have a disulfide bond connecting the cysteinyl residues at positions one and seven. The natural calcintonins include the salmon, eel, bovine, procine, ovine, rat and human calcitonins. Salmon calcitonin, for example, has the following formula: ##STR1##

In U.S. Pat. Nos. 3,926,938, 4,062,815, 3,929,758, 4,033,940, 4,336,187, 4,388,235 and 4,391,747 are disclosed improved syntheses of calcitonins including the salmon calcitonin referred to above.

We have discovered synthetic calcitonin analogs of the above-mentioned naturally occurring calcitonin peptides with 32 amino acids that have biological activity of the same type as the known calcitonins. A significant difference in structure is that in our new peptides the cysteines at positions one and seven are not connected by a disulfide bond. Instead, the sulfur atoms of the cysteines are protected with acetamidomethyl protecting groups. These new peptides are termed 1,7-Di-S-acetamidomethyl cysteinyl calcitonins. These new peptides have potencies comparable to the known calcitonins.

The formula of our new [1,7-Di-Cys(Acm)]salmon calcitonin having activity of the same type as known salmon calcitonin may be written as follows: ##STR2##

The formula of our new [1,7-Di-Cys(Acm)eel calcitonin may be written as follows: ##STR3##

As may be seen from the formula given above, 32 amino acids are involved and in this formula the positions are numbered according to the accepted procedure beginning at position 1 for the Cys on one end of the chain, and ending with Pro at position 32 at the other end of the chain. For clarity of description, this same numbering system will be followed in referring to the cycles of the synthesis. The assembly of the amino acids begins with cycle 32 which involves the coupling of proline and continues with cycle 31 which involves the coupling of threonine, etc.

In general we use a solid phase methodology of synthesis and start with a resin called benzhydrylamine resin (BHA resin). This resin is derived from a cross-linked polystyrene bead resin manufactured by copolymerization of styrene and divinylbenzene. Resin of this type is known and its preparation is further demonstrated by Pietta et al. [Pietta, P. S. and Marshall, G. R., Chem. Commun., 650 (1970)], and Orlowski et al., [J. Org. Chem., 41, 3701 (1976)]. The cross-linked polystyrene BHA resin is available from chemical supply houses. We use the designation ##STR4## to represent the BHA resin in which P is the polystyrene portion of the resin.

In this synthesis the amino acids are added one at a time to the insoluble resin until the total peptide sequence has been built up on the resin. The functional groups of the amino acids are protected by blocking groups. The .alpha.-amino group of the amino acids is protected by a tertiary butyloxycarbonyl group or an equivalent thereof. This .alpha.-tertiary butyloxycarbonyl group we designate as BOC. The hydroxyl functions of serine and threonine are protected by a benzyl or benzyl derivative group such as 4-methoxybenzyl, 4-methylbenzyl, 3,4-dimethylbenzyl, 4-chlorobenzyl, 2,6-dichlorobenzyl, 4-nitrobenzyl, benzhydryl or an equivalent thereof. We use the term Bz to represent the benzyl or benzyl derivative group.

The hydroxyl function of tyrosine may be unprotected, may be protected by a benzyl or benzyl derivative group as described above, as a Bz group, or may be protected by a benzyloxycarbonyl or a benzyloxycarbonyl derivative such as 2-chlorobenzyloxycarbonyl or 2-bromobenzyloxycarbonyl group or equivalent thereof. We use the term W to represent either no protective group, a Bz group, a benzyloxycarbonyl group or a benzyloxycarbonyl derivative group.

The thiol function of cysteine is protected by the acetamidomethyl protecting group. We use the character A to represent the acetamidomethyl group. The guanidine function of arginine may be protected by a nitro group, a tosyl group or an equivalent thereof. We use the character T to represent either a nitro group or a tosyl group. The .epsilon.-amino function of lysine may be protected by a benzyloxycarbonyl group or a benzyloxycarbonyl derivative such as a 2-chlorobenzyloxycarbonyl, 3,4-dimethylbenzyloxycarbonyl, or equivalents thereof. We use the character V to represent benzyloxycarbonyl group or a benzyloxycarbonyl derivative group. The protective groups used on the imidazole nitrogen of histidine are the benzyloxycarbonyl group and benzyloxycarbonyl derivatives such as described above for lysine and are designated as V. The .UPSILON.-carboxylic acid group of glutamic acid is protected by a benzyl or benzyl derivative group such as described for the protection hydroxyl function of serine and threonine. These protection groups are represented by the character Bz.

Preferred amino acid reactants for use in each of the 32 cycles of the synthesis of salmon calcitonin (used for exemplification only) are given in the following Table I:

Each of the amino acid derivatives mentioned in Table I may be purchased from supply houses.

The reaction vessel used in all steps of the resin peptide synthesis may be a glass vessel equipped with inlet ports at the top for addition of materials and a sintered glass disk at the bottom for removal of soluble reaction mixtures and wash solvents by filtration. Filtration may be performed either by vacuum or the use of nitrogen pressure. The contents of the vessel may be agitated by shaking the entire vessel or by mechanical stirrer.

In cycle 32 the BHA resin is placed in the reaction vessel and suspended in a solvent such as methylene chloride, chloroform, dimethylformamide, benzene or equivalents thereof in proportions of about 3 to 12 ml. of solvent per gram of resin. To this is added BOC-L-proline in an amount of about 1 to 6 equivalents per free amine equivalent of the BHA resin employed. After a period of mixing of 5 to 10 minutes, a coupling reagent (CA) such as dicyclohexylcarbodiimide (DCC) may be added, or other diimide coupling agents may be used. The diimide coupling agent may be used in the amount of 0.5 to 2.0 equivalents per equivalent of BOC-L-proline used.

The BOC-L-proline may be coupled in the absence of a coupling reagent if its active ester derivative, its azide derivative, its symmetrical anhydride derivative, or a suitably chosen mixed anhydride derivative is used. Active ester derivatives that may be employed are 2-nitrophenyl ester, 4-nitrophenyl ester, pentafluorophenyl ester, N-hydroxysuccimide ester or equivalents thereof. The active esters are used in amounts of 1 to 10 equivalents per free amine equivalent of BHA resin.

The reaction mixture consisting of the BHA resin, the solvent, the BOC-L-proline, and the coupling reagent or BOC-L-proline active ester is stirred or shaken mechanically until the reaction is complete as indicated by a ninhydrin test [E. Kaiser, et al., Anal. Biochem., 34, 595-8 (1970)] on a test sample. After completion of the coupling reaction, the BOC-L-proline resin may be washed with solvents such as methylene chloride, chloroform, methyl alcohol, benzene, dimethylformamide, or acetic acid. The amount of wash solvent may suitably be 5 to 20 ml. of solvent for each gram of BHA resin used initially. If it is desired to terminate the coupling reaction before completion, the washing procedure may be used and the remaining free amino groups on the BOC-L-proline resin may be blocked from further reaction by acetylation with an excess of acetylation reagents. The acetylation procedure may be performed by agitating the BOC-L-proline resin with a solution of the acetylation reagent for a period of 0.5 to 12 hours. Acetylation reagents such as N-acetylimidazole in methylene chloride solution or a mixture of acetic anhydride and triethylamine in chloroform may be used. The acetylation reagent may be used in the amount of 0.5 to 5.0 equivalents per equivalent of free amine titer of the starting BHA resin.

The coupling reaction to produce the BOC-L-proline resin may be described by the following formula: ##STR5##

The BOC-L-proline resin produced as above described may be washed with a solvent such as referred to above and deprotected by agitating it with an agent such as a mixture of trifluoroacetic acid (TFA) in a solvent such as methylene chloride, chloroform, benzene or equivalents thereof. The amount of TFA in the solvent may vary from 10 to 100% of the mixture. The amount of TFA-solvent mixture may vary from 3 to 20 ml. per gram of BHA resin used initially. The reaction time may be from about 10 minutes to 4 hours. The deprotection step is terminated by filtration to remove the TFA-solvent mixture. The residual TFA may be removed from the L-proline resin by washing with 3 to 20 ml. per gram of BHA resin of a 5 to 30% of triethylamine solution in a solvent such as methylene chloride, chloroform, benzene or equivalents thereof. Other tertiary or secondary organic amines may be used in place of the triethylamine, such as, trimethylamine, N-ethylpiperidine, diisopropylamine or equivalents thereof. The free amine titer of the L-proline resin may be determined by the Dorman titration procedure [Dorman, L. C., Tetrahedron Letters, 1969, 2319-21]. The deprotection reaction may be described by the following formula: ##STR6##

The prolyl-BHA resin obtained as a result of cycle 32 may be suspended in a coupling solvent, BOC-O-Bz-L-threonine added and the mixture equilibrated in the same manner. The coupling agent, DCC, may be added, and after completion of the reaction as indicated by the isatin test [E. Kaiser, et al., Anal. Chem. Acta, 118, 149-51 (1980)], the reaction mixture may be removed from the BOC-O-Bz-threonylprolyl-BHA resin by filtration. The peptide resin may be washed with solvents. The amounts of reactants and solvents and reaction times may be the same as described in cycle 32. The BOC group may be removed from the peptide resin by the deprotection method described in the cycle 32. The resulting O-Bz-threonylprolyl-BHA resin is then ready for cycle 30. The reactions of the cycle 31 may be shown by the following formula: ##STR7##

For convenience, we may write this resulting resin peptide using abbreviated nomenclature as follows: ##STR8##

In cycle 30, the coupling reaction and also the deprotection reaction may be performed in the same manner as in cycle 31 except that BOC-glycine is used in place of BOC-O-Bz-L-threonine. The reaction through coupling and deprotection may be written: ##STR9##

In cycle 29, the coupling and deprotection reactions may be performed in the same manner as in cycle 31 except for the substitution of BOC-O-Bz-L-serine as the amino acid derivative. This may be written: ##STR10##

In cycle 28, the coupling and deprotection reactions are performed as described in cycle 31 except that BOC-glycine is substituted as the amino acid reactant. These reactions through coupling and deprotection may be written as follows: ##STR11##

In this cycle, the coupling and deprotection reactions may be as in cycle 31 usng the same amino acid reactant, resulting in the following compound: ##STR12##

In cycle 26, the coupling reaction is performed using an active ester derivative of BOC-L-asparagine. The active ester procedure is used in place of the DCC coupling agent with BOC-L-asparagine or BOC-L-glutamine. The reaction using the active ester derivative of BOC-L-asparagine may be performed in the amount of 2 to 10 equivalents per free amine equivalent of BHA resin in dimethylformamide, mixtures of dimethylformamide with benzene, methylene chloride or chloroform or with equivalents thereof in the amount of 2 to 20 ml. of solvent per gram of BHA resin used initially.

Reaction times range from 1 to 72 hours. The reaction mixture may be removed from the BOC peptide resin by filtration after completion of the reaction as indicated by a ninhydrin test. The active ester derivatives employed may be 2-nitrophenyl esters, 4-nitrophenyl esters, pentafluorophenyl esters, or equivalents thereof. We use AE to designate the active ester portion of the derivative. The coupling reaction may be written: ##STR13##

The deprotection reaction to remove the BOC group is performed as in cycle 32.

In each of cycles 25 to 21, the coupling and deprotection reactions may be conducted using the methods and proportions of reactants as in cycle 31, using BOC-O-Bz-L-threonine in cycle 25, BOC-.omega.-T-L-arginine in cycle 24, BOC-L-proline in cycle 23, BOC-W-L-tyrosine in cycle 22, and BOC-O-Bz-L-threonine in cycle 21. The compound resulting from the completion of cycle 21 may be written: ##STR14##

In cycle 20, the coupling and deprotection reactions may be performed using the methods and proportions of reactants as in cycle 26 using a BOC-L-glutamine active ester derivative as the amino acid derivative, resulting in the compound: ##STR15##

In cycle 19, the reactions are performed as in cycle 30 using BOC-L-leucine as the amino acid derivative. The compound resulting from cycle 19 is: ##STR16##

In cycle 18, we may use as the amino acid derivative BOC-.epsilon.-V-L-lysine. Otherwise, cycle 18 methods may be performed as in cycle 31 resulting in the compound: ##STR17##

Cycles 17 to 15 may be performed as in cycle 31 except for the use of BOC-N(im)-V-L-histidine in cycle 17, BOC-L-leucine as the reactant in cycle 16 and BOC-L-glutamic acid Bz ester (Bz represents the same groups as it represents for serine and threonine) as the reactant in cycle 15, resulting in the following compound from cycle 15: ##STR18##

Cycle 14 may be performed identically to cycle 20 using BOC-L-glutamine-AE as the amino acid derivative. Cycles 13 to 8 may be performed as in cycle 31 except for the use of BOC-O-Bz-L-serine in cycle 13, BOC-L-leucine in cycle 12, BOC-.epsilon.-V-L-lysine in cycle 11, BOC-glycine in cycle 10, BOC-L-leucine in cycle 9, and BOC-L-valine in cycle 8 resulting in the compound: ##STR19##

Cycle 7 may be performed as in cycle 31 except for the use of BOC-S-acetamidomethyl-L-cysteine for the amino acid derivative. The compound resulting from cycle 7 is described by the formula: ##STR20##

Cycles 6 to 4 may be performed as in cycle 31 except that BOC-O-Bz-L-threonine be used as the amino acid derivative in cycle 6, BOC-O-Bz-L-serine may be used as the amino acid derivative in cycle 5 and cycle 2, and BOC-L-leucine may be used in Cycle 4 as the amino acid derivative. Cycle 3 may be performed identically to cycle 26 using BOC-L-asparagine active ester. The compound resulting from cycle 2 is: ##STR21##

This cycle is performed identically to cycle 7 using BOC-S-acetamidomethyl-L-cysteine derivatives. The compound resulting from cycle 1 are illustrated by the formula: ##STR22##

Cycle 1 represents the completion of the resin peptide. The resin peptide may be removed from the reaction vessel and dried in a vacuum. The weight of the resin peptide may be expected to be from 2.0 to 3.5 times the weight of BHA resin used initially in the synthesis.

The peptide is cleaved from the resin peptide resulting from cycle 1 by treatment with liquid hydrogen fluoride (HF). The HF cleavage reaction may be performed by treating a mixture of the resin peptide and anisole (0.5 to 5 ml. for each gram of resin peptide) with liquid HF (2 to 20 ml. for each gram of resin peptide) for 0.5 to 20 hours at -20 degrees to +15 degrees centigrade. After the reaction period, the excess HF may be removed by evaporation and the resulting mixture of peptide and resin beads may be extracted with organic solvent such as ethyl acetate, diethyl ether, benzene or the like to remove the anisole and residual HF. The peptide may be separated from the resin beads by extraction into aqueous acetic acid. The peptide at this stage is not cyclic but is the non-cyclic product without the disulfide bond between the cysteines at positions 1 and 7 in the molecule.

The HF treatment removes all blocking groups from the peptide, except the S-acetamidomethyl blocking groups on the thiol function of cysteine residue at both position 1 and 7. The S-acetamidomethyl group is stable to the HF cleavage procedure and remains intact throughout the cleavage and extraction procedures.

Thus, the peptides obtained after HF cleavage is of the following type. ##STR23## Any calcitonin so synthesized is designated [1,7-Di-Cys(Acm)]-calcitonin.

The crude peptide solutions at pH 5.0 from the above synthesis may be concentrated using an ion-exchange procedure. The concentrate may be purified by a combination of gel-filtration procedures, ion-exchange chromatography methods and partition chromatography. The final purified product may be obtained from solution by freeze-drying as a fluffy white solid. The product gives the correct amino acid analysis for the desired peptide.

Following is the specific example of the preparation of the peptide.

The BHA resin (5 g.) with an amine titer of 0.61 meq./g. was placed in the reacter vessel of a peptide synthesizer marketed by Vega Biochemicals of Tucson, Ariz. The resin was treated with 25 ml. of the following solvents filtering after each treatment:

Methylene chloride for 2 minutes

Chloroform for 2 minutes two times each

10% triethylamine in chloroform for 5 minutes two times each

Chloroform for 2 minutes

Methylene chloride for 2 minutes three times each

Coupling:

The BHA resin, 25 ml. of methylene chloride and 1.31 g. (0.0061 moles) of BOC-L-proline was stirred for 10 minutes. 6.1 ml. of methylene chloride solution of dicyclohexylcarbodiimide (1 milliequivalent of DCC per 1 ml. of solution) was added to the reactor and the mixture agitated for 6 hours. The reaction mixture was removed from the reactor by filtration and the BOC-prolyl BHA resin was subjected to the following successive 2 minute, 25 ml. washes, removing the wash by filtration each time:

Methylene chloride two times

Methyl alcohol two times

Methylene chloride three times

Acetylation:

The resin was then agitated with a mixture of 1.5 ml. of triethylamine (TEA) 1 ml. of acetic anhydride and 25 ml. of chloroform for 2 hours. The reaction mixture was removed by filtration and the resin subjected to the following 2 minute, 25 ml. washes:

Chloroform two times

Methyl alcohol two times

Methylene chloride three times

Deprotection:

The BOC-protected resin was agitated for 5 minutes with a mixture of 15 ml. of trifluoroacetic acid (TFA) and 15 ml. of methylene chloride. This mixture was removed by filtration and the resin was agitated with a second mixture of 15 ml. of TFA and 15 ml. of methylene chloride for 30 minutes. The reaction mixture was removed by filtration and the resin subject to the following 25 ml. washes:

Methylene chloride two times two minutes each

Methyl alcohol two times two minutes each

Chloroform two times two minutes each

10% TEA in chlorofom two times ten minutes each

Chloroform two times two minutes each

Methylene chloride two times two minutes each

The L-proline BHA resin was titrated to establish the amine or proline titer. This value was 0.55 milliequivalents of amine or proline per gram of resin.

Coupling:

The L-prolyl resin, 25 ml. of methylene chloride and 1.64 g. (0.0053 mole) of BOC-O-benzyl-L-threonine were agitated for 10 minutes. Then 5.5 ml. of methylene chloride solution of dicyclohexylcarbodiimide (1 milliequivalent of DCC per 1 ml. of solution or a total of 0.0055 mole of DCC) was added to the reactor and the mixture agitated for 2 hours. The reaction mixture was removed from the reactor and the resin was subjected to the following successive 2 minute, 25 ml. washes, removing the wash by filtration each time.

Methylene chloride two times

Methyl alcohol two times

Methylene chloride three times

An isatin was negative.

Deprotection:

The deprotection procedure described in cycle 32 was repeated for this cycle.

The coupling and deprotection procedures used in these cycles were the same as in cycle 31 except that the following amino acid derivatives were used in place of the threonine derivative:

Cycle 30: 0.93 g. (0.0053 mole) of BOC glycine

Cycle 29: 1.55 g. (0.0053 mole) of BOC-O-benzyl-L-serine

Cycle 28: The reactant used was the same as cycle 30

Cycle 27: The reactant used was the same as cycle 31

Coupling:

The peptide resin obtained from cycle 27 was washed twice with 25 ml. portions of dimethylformamide (DMF). The resin was then agitated for 24 hours with a solution of 2.82 g. (0.008 mole) of BOC-L-asparagine p-nitrophenyl ester in 35 ml. of DMF. The reaction mixture was filtered and the resin peptide subjected to two minute washes with two successive 25 ml. portions of the following solvents: DMF, methylene chloride, methanol, methylene chloride. Each individual solvent was removed by filtration. A ninhydrin test was negative.

Deprotection:

The deprotection procedure used in cycle 32 was repeated.

Coupling and deprotection procedures were the same as in cycle 31 using the same reactants and amounts.

Coupling:

The resin peptide obtained from cycle 25 was washed with two successive 25 ml. portions of DMF. The resin peptide was then agitated for 10 minutes with a mixture of 3.42 g. (0.008 mole) of BOC-N-.omega.-tosyl-L-arginine and 25 ml. of DMF. Then 8 ml. of DCC in methylene chloride (equivalent to 0.008 mole of DCC) was added and the mixture agitated for 6 hours. The reaction mixture was removed by filtration. The resin peptide was subjected to two minute washes with two successive 25 ml. portions of the following solvents: DMF, methylene chloride, methyl alcohol, methylene chloride. The ninhydrin test was negative.

Deprotection:

The deprotection used in cycle 32 was repeated.

Coupling:

The peptide resin obtained from cycle 24 was agitated for 10 minutes with 1.72 g. (0.008 mole) of BOC-L-proline and 25 ml. of methylene chloride. 8 ml. of DCC in methylene chloride (equivalent to 0.008 mole of DCC) was added and the mixture agitated for 6 hours. The reaction mixture was removed by filtration and the resin peptide subjected to two minute washes with two successive 25 ml. portions of the following solvents: methylene chloride, methyl alcohol, methylene chloride. Each individual was was removed by filtration. The ninhydrin test was negative.

Deprotection:

The deprotection used in cycle 32 was repeated.

The coupling and deprotection procedures used in this cycle was the same as in cycle 23 except that in the coupling reaction the following amino acid derivative was used in place of BOC-L-proline.

Cycle 22: 4.07 g. (0.008 mole) BOC-O-2-bromobenzyl-oxycarbonyl-L-tyrosine

Cycle 21: 2.47 g. (0.008 mole) BOC-O-benzyl-L-threonine

This procedure is the same as cycle 26 except that 2.94 g. (0.008 mole) of BOC-L-glutamine p-nitrophenyl ester is used in place of the asparagine derivative.

The procedure is the same as used in cycle 30 except that the following amino acid dervatives were used in place of the threonine derivative:

Cycle 19: 1.32 g. (0.0053 mole) of BOC-L-leucine

Cycle 18: 2.20 g. (0.0053 mole) of BOC-.epsilon.-2-chloro benzyloxy-L-lysine

Cycle 17: 2.06 g. (0.0053 mole) of BOC-N(im)-carbobenzyloxy-L-histidine

Cycle 16: 1.32 g. (0.0053 mole) of BOC-L-leucine

Cycle 15: 1.79 g. (0.0053 mole) of BOC-L-glutamic acid-.UPSILON.-benzyl ester

Same as cycle 20.

The procedure used was the same as was used in cycle 23 except that in the coupling reaction 2.36 g. (0.008 mole) of BOC-O-benzyl-L-serine was used in place of the proline derivative.

The procedures used were the same as used in cycle 31 except in the coupling reactions the following amine acid derivatives were used in place of threonine derivative.

Cycle 12: Same reactants as used in cycle 19

Cycle 11: The reactants were the same as in cycle 18

Cycle 10: Same reactants as used in cycle 30

Cycle 9: Same reactants as used in cycle 19

Coupling:

The resin peptide from cycle 9 was agitated for 10 minutes with 1.74 g. (0.008 mole) of BOC-L-valine and 25 ml. of methylene chloride. Then 8 ml. of DCC in methylene chloride (equivalent to 0.008 mole of DCC) was added and the mixture agitated for 16 hours. The reaction mixture was removed by filtration. The resin peptide was subjected to two minute washes with two successive 25 ml. portions of the following solvents: methylene chloride, methyl alcohol, methylene chloride. Each individual wash was removed by filtration.

Deprotection:

See cycle 31.

The procedure was the same as used in cycle 31 except that in the coupling reaction 1.59 g. (0.0053) of BOC-S-acetamidomethyl-L-cysteine was used in place of the threonine derivative.

The reactants and procedures used were the same as cycle 31.

The reactants and procedures used were the same as cycle 29.

The reactants and procedures used were the same as cycle 19.

The reactants and procedures used were the same as cycle 26.

The reactions and procedures used were the same as cycle 29.

The reactants and procedures used were the same as cycle 7.

After completion of cycle 1, the resin peptide was washed with two successive 25 ml. portions of n-hexane. The peptidic material was removed from the reactor and dried in an vacuum oven at 40 degrees centigrade and 0.1 mm. of Hg for 24 hours.

The dried resin peptide (2 g.) and 2 ml. of anisole were placed in a Teflon reaction vessel. The vessel equipped with a Teflon-coated magnet stirrer was placed in a dry ice-acetone bath and 15 ml. of hydrogen fluoride gas was condensed into the vessel. This mixture was stirred at 0 degrees centigrade in an ice bath for 1 hour. The hydrogen fluoride was removed by evaporation at reduced pressure. The residue was triturated with six 25 ml. portions of ethyl acetate. The peptide was extracted from the resin beads with 120 ml. of 0.1 molar aqueous acetic solution.

The 200 ml. of solution from the above synthesis at pH 5.0 was concentrated using a SP-25 ion-exchange column. The 25 ml. concentrate removed from the column with 0.7 molar sodium chloride solution was desalted and purified by passing through a Sephadex G-25 (fine) gel-filtration column and eluting with 0.03 molar aqueous acetic acid solution. The [1,7-Di-Cys(Acm)]SCT fraction from this column was adjusted to pH 6 by the addition of ammonium hydroxide solution. This solution was further purified by ion-exchange chromatography using a Whatman CM-52 column eluted with ammonium acetate buffer. The [1,7-Di-Cys(Acm)]SCT fraction from this column was adjusted to pH 5.0 by addition of glacial acetic acid. This solution was concentrated using a SP-Sephadex C-25 ion-exchange column. The 30 ml. concentrate removed from the column with 0.7 molar sodium chloride solution was desalted with a Sephadex G-25 (fine) gel-filtration column. The peptide fraction was collected and freeze-dried. The product was further purified by partition chromatography using a Sephadex G-25 fine column and the solvent system: n-butanol, ethanol, 0.2N ammonium acetate containing 0.04% acetic acid (4-1-5). The product elutes from the column at an Rf value of 0.41. The fractions containing the product were combined and the n-butanol removed by evaporation. The product was recovered by lyophilization. The solid was then gel-filtered on a Sephadex G-25 (fine) column with 0.2M acetic acid solution. The purified peptide fraction was collected and lyophilized.

The product was obtained as a fluffy white solid. Amino acid analysis of the product after acid hydrolysis in the presence of phenol gave the following ratios with the theoretical values given in parenthesis: Asp 2.0 (2), Thr 5.2 (5), Ser 3.8 (4), Glu 2.8 (3), Pro 2.0 (2), Gly 3.0 (4), Val 0.9 (1), Leu 5.0 (5), His 0.92 (l), Lys 1.9 (2), Arg 0.94 (1), Cys 1.91 (2), Tyr 0.91 (1).

The biological potency of [1,7-Di-Cys(Acm)]salmon calcitonin was determined by comparing the reduction of serum calcium concentration which followed administration by graded doses of [1,7-Di-Cys(Acm)]SCT and synthetic salmon calcitonin standard. Rats were divided into four groups of seven animals, and each group was assigned at random to a dose of standard or test solution. Low and high doses were chosen from the linear portion of the dose-response curve. For the salmon calcitonin standard, the values were 0.7 and 2.1 mcg. peptide/100 g. body weight (BW). This dose approximates 3 and 9 MU/100 g. BW. Peptides were given by subcutaneous injection (0.2 ml/100 g. BW) and blood was withdrawn 1 hour later for serum calcium determination. Sera are processed and analyzed within two hours of collection. Results were analyzed within two hours of collection. Results were analyzed by a 2.times.2 parallel line assay [Gaddum, J. H., J. Pharm. Pharmacol., 6, 345 (1953)]. The standard salmon calcitonin used was independently determined to contain greater than 4,000 IU/mg. [1,7-Di-Cys(Acm)]SCT assayed at 4250 IU/mg.

While only certain embodiments of our invention have been described in specific detail it will be apparent to those skilled in this art that many other specific embodiments may be practiced and many changes may be made, all within the spirit of the invention and the scope of the appended claims.