WO2016038445A1

WO2016038445A1 - Injector pens

Info

Publication number: WO2016038445A1
Application number: PCT/IB2015/001606
Authority: WO
Inventors: Alexander AMBRIDGE; Barry Knight
Original assignee: Shaily Medical Plastics Private Limited
Priority date: 2014-09-12
Filing date: 2015-09-14
Publication date: 2016-03-17
Also published as: GB201416149D0; GB2530076B; GB2530076A

Abstract

An injector pen for setting and delivering doses of a drug from a cartridge 8 is characterized by a manual dose selector 4 that slides axially, without rotation, relative to a body 2 of the pen. An arrangement of concentric cylinders 30,28,24 incorporating a non-locking thread 29,31 couples the dose selector 4 to a piston rod 16 having an external helical thread 19. Means 70,76,78 may be provided for preventing a further dose from being set by the selector 4 if less than a full dose of drug remains in the cartridge 8. Means 20,60 may further be provided to facilitate resetting the piston rod 16 to its starting position after the used drug cartridge 8 has been removed from the pen.

Description

TITLE

Injector pens

DESCRIPTION

Technical field

An injector pen provides a mechanism for setting a desired dose of a drug and delivering that dose to a patient from a drug cartridge. The drug cartridge typically contains multiple doses of the drug in a syringe and the injector pen comprises a piston rod that is progressively extended from the pen to operate the syringe, the piston rod moving through a distance suitable for the dose required to be delivered. Aspects of the present invention are particularly suitable for a pen that accommodates a disposable drug cartridge, whereby the mechanism of the pen can be re-used when the drug cartridge is replaced.

Background of the invention

Many designs of injector pen are known. Typically the pen has cylindrical pen body and a dose selector that in a dose setting mode can be moved manually relative to the pen body to an axial position that determines the dose for delivery, then in a drug delivery mode can be pushed into the pen body to deliver the dose. The pen further comprises means for attaching a drug cartridge to the pen; and a piston rod along the axis of the pen, which acts on the syringe of the drug cartridge. A button on the end of the pen is pressed to engage the drug delivery mode and push the dose selector into the pen body. During the drug delivery mode, a drive mechanism within the pen body converts the axial movement of the dose selector into axial movement of the piston rod through a distance suitable to deliver the required dose from the cartridge. The distance moved by the piston rod is thus determined by the distance moved by the dose selector but the two distances need not be equal. On each use of the pen with the same cartridge, the piston rod advances further along the axis. An example of such an injector pen is illustrated in patent application EP 1681070 A.

In the drug delivery mode, the dose selector makes a purely sliding movement, i.e. translation along the axis without simultaneous rotation. In the dose setting mode, the movement of the dose selector follows a helical path so that it is screwed out of or back into the pen body. Because distance along the helical path is much longer than the corresponding distance along the axis, the range of possible doses may be finely resolved and the desired dose precisely determined. However, many injector pens are used repeatedly to deliver the same dose - perhaps by a single patient undergoing a course of regular treatment - or to deliver only a limited range of doses, whereby fine resolution is not required. The unscrewing action may be difficult to accomplish by users with poor manual dexterity and the fine resolution may make dose selection difficult for users with impaired vision. Even for fully able users, if they have a requirement for only a small range of possible doses it is desirable that the pen should be correspondingly simple to operate.

Because the injector pen can deliver multiple doses from a single drug cartridge, it is likely that after a period of use the cartridge will reach a state where it has less than a full dose of the drug remaining. Safety regulations require that the user must be made aware that the pen is unable to deliver a full dose on its next operation. If the design of the pen does not permit the amount of the remaining, partial dose to be precisely determined by the user, the pen should be locked out from delivering the partial dose at all.

If the pen is to be used with replaceable drug cartridges then, because the piston rod advances along the axis with each use of the pen, there must be provided a means for resetting the position of the piston rod after one drug cartridge has been removed and before the next one is attached.

Summary of the invention

The invention provides an injector pen having a sliding dose selector, as defined in claim 1.

The invention further provides a method of operating an injector pen with a sliding dose selector, as defined in claim 12. The invention also provides an injector pen having a reset brake, as defined in claim 20.

The invention further provides a method of resetting an injector pen, as defined in claim 28.

Preferred but non-essential features of the invention are defined in the dependent claims.

Drawings

Figure 1 is a longitudinal cross-section of an injector pen in accordance with a first embodiment of the present invention.

Figure 2 is a longitudinal cross-section of the injector pen of Figure 1, taken on a plane perpendicular to that of Figure 1.

Figure 3 is an enlargement of Figure 1 showing just the first end of the injector pen. Figure 4 is an enlargement of Figure 2 showing just the first end of the injector pen. Figures 5A to 5D together form an exploded, perspective view of the injector pen of Figures 1 to 4.

Figure 6 is a schematic illustration of a second embodiment of injector pen in accordance with the invention, comprising an alternative reset mechanism and a last dose lock-out mechanism.

Figure 7 is a cross-section on line A-A of Figure 6. Description of preferred embodiments of the invention

An injector pen according to a first embodiment of the injection is illustrated in Figures 1 to 5. It comprises a generally cylindrical, hollow pen body 2 centred on a longitudinal axis of the pen. A sliding dose selector 4 is mounted within the pen body 2 and projects from a first end of the pen body 2 to expose an outer surface 5 of the dose selector 4 that can be gripped manually, e.g. between a finger and thumb of a user. The first end of the pen terminates in an activation button 6.

A disposable drug cartridge 8 is removably mounted on a second end of the pen body 2, for example by a threaded connection 9. The drug cartridge 8 comprises a syringe 10 that contains multiple doses of a drug or other biologically active substance for injection into a patient. The drug cartridge 8 further comprises means for mounting a hypodermic needle 12 in fluid communication with the syringe 10 and a piston 14 that can be pushed along the axis of the syringe 10 to force doses of the drug to be ejected from the needle 12. In Figs. 1 and 2 the piston is shown twice: both at its start position (labelled 14) and at its end position (labelled 14'). The hypodermic needle 12 may be covered by a needle cap 13 to protect the needle 12 from damage and to protect the user of the injector pen from accidental injection. The drug cartridge 8 is enclosed by a cover 11 that provides the threaded connection 9 for mounting the drug cartridge 8 to the pen body 2. The cover 11 is preferably at least partially transparent so that the position of the piston 14 within the cartridge 8 can be seen. The drug cartridge 8, cover 11 , needle 12 and needle cap 13 may be protected by a cartridge cap 15 when the pen is not in use. The cartridge cap 15 may be supplied as a matching accessory with the pen.

The hypodermic needle 12 is designed to be replaced after each use of the pen. The drug cartridge 8 is designed to be disposed of when its syringe 10 is empty (or when less than a whole dose of the drug remains). The clear cover 11 and the cartridge cap 15 are designed to be kept for re-use with successive drug cartridges 8. Such drug cartridges with disposable hypodermic needles are well known and do not form part of the present invention. Their details are not important provided they can be removably mounted in an appropriate fashion on the pen body 2 and they can be operated by pushing the piston 14 along the axis.

The injector pen comprises a piston rod 16 that lies along the axis, partly lying within the pen body 2 and partly extending from the second end of the pen body 2 to penetrate the syringe 10 of an attached drug cartridge 8. By moving along the axis in a direction from the first end towards the second end of the pen body, an end 17 of the piston rod 16 pushes the piston along the axis to operate the syringe 10 and deliver a dose of the drug to the patient.

The piston rod 16 is solid in cross-section and carries a helical thread 19 on its outer surface. The piston rod 16 also comprises an opposing pair of keyway channels 18 extending along most of its length. As best seen in Figure 4, a piston guide 20 is mounted concentrically about the axis at the second end of the pen body 2. The piston guide 20 carries a restraint in the form of two inwardly- facing keys 22 that are received in the keyways 18 of the piston rod 16. This arrangement prevents relative rotation between the piston rod 16 and the piston guide 20, while permitting the piston rod 16 to slide along the axis relative to the piston guide 20. Forms of non-circular cross- section other than a pair of keys 22 and keyways 18 could be used to achieve the same result, for example a single key and keyway or a pair of flats. In the illustrated embodiment the piston guide 20 is held fixed relative to the pen body 2 while the pen is used to set and deliver a dose so the restraint 22 constrains the piston rod 16 to slide axially without rotation during normal use.

The axial movement of the piston rod 16 is driven by a drive cylinder 24 that is coaxially mounted in the pen body 2 around the piston rod 16 so as to be capable of rotation but not axial translation. An inner helical thread 25 on the drive cylinder 24 engages the outer helical thread 19 on the piston rod so that rotation of the drive cylinder 24 drives axial movement of the piston rod 16. A ratchet 26 is provided between the drive cylinder 24 and the pen body 2 to permit rotation of the drive cylinder 24 in only one direction, namely the direction that causes the piston rod 1 to project further from the second end of the pen body 2 and deliver a dose of drug from the syringe.

A drive nut 28 is attached to the drive cylinder 24 to move integrally therewith. The drive nut 28 may alternatively be formed integrally with the drive cylinder 24. On an outer surface of the drive nut 28 is formed one part 29 of a non-locking thread. A dose delivery tube 30 surrounds the drive cylinder 24 and the drive nut 28. On an inner surface of the dose delivery tube 30 is formed the other part 31 of the non-locking thread. When the dose delivery tube 30 slides axially relative to the drive nut 28 in a direction towards the second end of the pen body 2, the engagement of the two parts of the non-locking thread 29,31 acts to drive rotation of the drive nut 28 and drive cylinder 24. The dose delivery tube 30 cannot slide axially in the opposite direction because counter-rotation of the drive cylinder 24 is prevented by the ratchet 26. The dose delivery tube 30 is surrounded by the aforementioned sliding dose selector 4. The sliding dose selector 4 is mounted in the pen body 2 such that it can only slide axially and cannot rotate relative to the pen body. A shoulder 32 and clip 33 constrain the dose delivery tube 30 to move together with the sliding dose selector 4 in the axial direction but in certain circumstances, explained below, the dose delivery tube 30 is also permitted to rotate relative to the sliding dose selector 4. Indicia 34 are provided on an outer surface of the dose selector 4. A window 35 in the pen body 2 permits an appropriate one of the indicia to be viewed as the dose selector 4 moves axially relative to the pen body 2 in order to provide a visual indication of the dose that is to be delivered. Detents 36 may also be provided, which act between the dose selector 4 and the pen body 2 to provide slight resistance to their relative movement at intervals along the axis in order to give a further, tactile indication of the dose to be delivered.

In the illustrated embodiment, the pen body 2 comprises a cylindrical outer wall 37 and a concentric, cylindrical inner wall 38 spaced from the outer wall 37. The inner wall 38 is nested between the drive cylinder 24 and the dose delivery tube 30 and extends along only part of their axial length so as not to interfere with the engagement of the nonlocking threads 29,31. The inner wall 38 of the pen body 2 provides strength and rigidity to the assembly of moving parts. In other embodiments the inner wall 38 may be nested instead between the dose delivery tube 30 and the dose selector 4. As a further alternative, if the various components are sufficiently rigid and the play between them is sufficiently small, the inner wall 38 of the pen body 2 may be omitted.

The activation button 6 is mounted at the first end of the pen so as to be retained by the pen body 2 but capable of a small amount of axial movement. The activation button 6 comprises an inner ring of axially facing teeth 42 that are capable of engaging with a ring of teeth 43 on the dose delivery tube 30. The activation button 6 further comprises an outer ring of axially facing teeth 40 that are capable of engaging with a ring of teeth 41 on the dose selector 4. As illustrated, each pair of mutually engaging rings of teeth may comprise one ring in which the teeth form a continuous array and one ring in which the teeth are spaced at intervals around the circumference. When pressure is applied to the end of the activation button 6, both sets of teeth 40,41; 42,43 are brought into engagement, whereby the activation button prevents relative rotation between the dose selector 4 and the dose delivery tube 30. As the dose selector 4 can always move only axially, in this drug delivery mode the dose delivery tube 30 is therefore also constrained to move only axially. When pressure is not applied to the end of the activation button 6, the respective sets of teeth 40,41; 42,43 are not forced into engagement and can disengage if any a rotational force is applied to them. In this dose setting mode the dose delivery tube 30 is therefore free to rotate relative to the axially moving dose selector 4.

In the dose setting mode, the user grips the outer surface 5 of the dose selector 4 and slides the dose selector axially relative to the pen body 2 to set the desired dose as indicated by the indicia 34 visible through the window 35. As the dose selector 4 moves axially, the dose delivery tube 30 is constrained by the shoulder 32 and clip 33 to move through the same axial distance. However, because pressure is not being applied to the activation button 6, the dose delivery tube 30 is free to rotate relative to the dose selector 4 and it does so, guided by the helical movement of the part of the non-locking thread 31 on the dose delivery tube 30 along the stationary part of the non-locking thread 29 on the drive nut 28. The drive nut 28 is held stationary by the action of the ratchet 26, which is designed to provide a resistance to rotation of the drive cylinder 24 in either direction that is greater than the resistance to rotation of the dose delivery tube 30 while the activation button 6 is not being pressed. The desired dose can therefore be selected without causing any movement of the piston rod 16.

In the drug delivery mode, the activation button 6 is pressed, typically by the thumb of the user, to push the dose selector 4 axially back into the pen body 2, towards the second end of the pen. Because of the pressure exerted on the activation button 6, the respective sets of teeth 40,41 ; 42,43 are engaged and the dose delivery tube 30 is unable to rotate relative to the dose selector 4. The dose delivery tube 30 therefore also slides purely axially back into the pen body 2 and acts through the non-locking thread 29,31 to overcome the resistance of the ratchet 26 and cause the drive nut 28 and drive cylinder 24 to rotate in the direction permitted by the ratchet. The piston rod 16 is prevented from rotating by the keys 22 of the piston guide 20 so, when the rotating inner helical thread 25 of the drive cylinder 24 acts on the stationary outer helical thread 19 of the piston rod 16, the piston rod is driven forwards along the axis. The distance moved by the end 17 of the piston rod 16 controls the dose that is delivered from the drug cartridge 8. It is determined by the axial distance moved by the dose selector 4 as it is returned fully into the pen body 2, scaled by a ratio that depends on the pitch of the non-locking thread 29,31 and the pitch of the helical thread 19,25.

According to the description so far, the only function of the piston guide 20 is to provide a restraint, in the form of keys 22, that is fixed relative to the pen body 2 in both the dose setting mode and the dose delivery mode. Thus the injector pen could function in both of those modes if the piston guide 20 was simply formed as part of the pen body 2. However, when the pen has undergone multiple uses so that the piston rod 16 protrudes from the pen body 2 to its maximum extent - or to the extent that there is less than a whole dose of the drug remaining - it is desirable that the drug cartridge 8 can be replaced and the piston rod 16 can be reset to its starting position so that the pen can be re-used. A second aspect of the invention facilitates that process.

The piston guide 20 may be provided by a reset element that comprises a fixed ring 44 and a rotatable hub 46. The fixed ring 44 is anchored against rotation relative to the pen body 2 by a pair of lugs 48 that project radially outwards through apertures in the pen body in the region of the thread 9 for attaching a drug cartridge 8. When no drug cartridge 8 is present, the lugs 48 project beyond the apertures. The rotatable hub 46 is located within the fixed ring 44 and, when no drug cartridge 8 is present, the hub 46 is free to rotate about the axis of the pen. When a drug cartridge 8 is attached to the thread 9, the lugs 48 are deflected radially inwards so that a set of inwardly facing teeth 50 on each lug engages a set of outwardly facing teeth 52 on the rotatable hub 46. The engagement between the teeth of the rotatable hub 46 and the teeth of the fixed ring 44 acts as a brake to lock the hub 46 and the associated piston guide 20 against rotation during use of the pen as previously described.

Upon removal of an old drug cartridge 8 by unscrewing it from the thread 9, the lugs 48 spring outwards to release the rotatable hub 46 and permit it to rotate about the axis. It is now possible to turn the hub 46 manually, which simultaneously turns the piston rod 16 because of the interaction between the keys 22 of the hub 46 and the keyways 18 of the piston rod 16. In this way, the piston rod 16 can be wound back towards the first end of the pen along the helical thread 25 of the drive cylinder 24 to reach its starting position. The drive cylinder 24 is prevented from rotating in the same sense by the ratchet 26.

In a variant of the illustrated embodiment, the locations of the restraint and the internal thread can be exchanged, i.e. the keys engaging the keyways 18 of the piston rod 16 can be provided on an inner surface of the drive cylinder 24, while the inner helical thread engaging the outer helical thread 19 of the piston rod 16 is provided on an inner surface of the piston guide 20. The operation of this variant is exactly as previously described except that during the drug delivery mode the piston rod 16 rotates with the drive cylinder 24 at the same time as it is driven forwards, by the action of the fixed helical thread on the piston guide 20.

Figure 6 is a schematic illustration of a second embodiment of injector pen in accordance with the invention. Like parts are numbered in the same way as the first embodiment and will not be explained again. This embodiment incorporates the variant just described, in which the restraint is formed by keys 56 formed on the inner surface of the drive cylinder 24, and the outer helical thread 19 of the piston rod 16 is engaged by an inner helical thread 58 on the piston guide 60. In this embodiment the piston guide 60 is again provided by a reset element. It does not comprise a separate, rotatable hub but only a fixed ring 64. The fixed ring 64 comprises lugs 66 similar to those of the first embodiment, except that the radially inner surface of each lug 66 carries a portion of the inner helical thread 58 for engagement directly with the piston rod 16. There are preferably three or four of the lugs 66 spaced angularly around the piston guide 60.

When a drug cartridge 8 is attached to the thread 9 of the pen body 2, the lugs 66 are deflected inwards so that their inner helical thread 58 engages the outer helical thread 19 of the piston rod and forces the piston rod 16 to travel along the axis as it is rotated during the drug delivery mode. Upon removal of an old drug cartridge 8 by unscrewing it from the thread 9, the lugs 66 spring outwards to release the helical thread of the piston rod 16. It is now possible to slide the piston rod 16 manually along its keyways 18, directly back towards the first end of the pen to reach its starting position. This will be a much easier and faster reset procedure than winding the piston rod 16 along its helical thread 19 in the first embodiment.

Because the injector pen can deliver multiple doses from a single drug cartridge, after repeated uses the piston rod will have advanced to a position near its maximum extension where the drug cartridge has less than a full dose of the drug remaining. The embodiment of the invention illustrated in Figures 6 and 7 further provides a lock-out mechanism to prevent the pen from delivering the partial dose. Although illustrated together in Figure 6, the reset mechanism and the last dose lock-out mechanism are substantially independent of one another and an injector pen within the scope of the invention may incorporate one of these mechanisms without the other.

As part of the lock-out mechanism the drive cylinder 24 incorporates a pair of deformable stop members 70 located on opposite sides of the axis. A radially inner part of each stop member 70 comprises a key 72 that is located in one of the keyways 18 of the piston rod 16. A radially outer part 73 of each stop member 70 extends into a slot 74 in the drive nut 28, remaining within the radius of the drive nut when the stop member 70 is not deformed. The inner cylindrical wall 38 of the pen body 2 is truncated to permit this to happen and, in order to maintain the rigidity of the assembly, the length of the drive nut 28 is extended to fill the void except where the slots 74 are formed in it.

On an inner surface of the dose delivery tube 30, aligned with and surrounding the stop members 70, is formed a ring of inwardly facing teeth 76. The outer extremity 73 of each stop member 70 is formed in a shape that complements the shape of the teeth 76. Preferably the teeth 76 in the ring have a saw-tooth profile to form a circular ratchet and the outer extremity 73 of each stop member 70 forms a pawl. The outer extremity 73 may comprise a flexible, inclined tab that, when the stop members 70 are deformed outwards to engage the ratchet, can ride over the teeth 76 of the ratchet in one direction (anti-clockwise in Figure 7) but prevent rotation in the other direction.

After many uses of the injector pen with a single drug cartridge 8, the piston rod 16 will have moved a long way in the direction of the second end of the pen, i.e. much further down than the position shown in Fig. 6. The axial location of the stop members 70 is such that, when the piston 16 has moved so far that only a single dose of drug remains in the cartridge 8, the keys 72 of the stop members encounter the ends 78 of the keyways 18. The end 78 of each keyway preferably takes the form of a cam surface so that as the piston rod 16 continues its onward movement - all the time rotating with the drive cylinder 24 and drive nut 28 in this embodiment - the cam surfaces 78 cause the stop members 70 to deform radially outwards so that the outer extremities 73 of the stop members engage the ring of teeth 76 on the dose delivery tube 30. Delivery of the present, final dose can be completed because the ratchet arrangement permits the drive cylinder 24 and drive nut 28 to continue rotating relative to the dose delivery tube 30, which does not rotate during the drug delivery mode. However, if the user attempts to use the pen to set a further dose, which the near-empty drug cartridge 8 will be unable to provide, the pen can no longer be operated in dose setting mode. That is because in dose setting mode the drive cylinder 24 is held stationary by the ratchet 26 on the pen body 2, therefore the dose delivery tube 30 can only move axially with the dose selector 4 if it is permitted simultaneously to rotate and follow the non-locking threads 29,31. Because of the engagement of the stop members 70 with the ring of teeth 76 on the dose delivery tube 30, the tube 30 cannot rotate and therefore it and the dose selector 4 cannot be moved axially. The user is forewarned of this situation because the engagement of the ratchet 73,76 of the lock-out mechanism will give an audible and tactile signal during delivery of the final dose.

Claims

1. An inj ector pen comprising:

a pen body (2);

a piston rod (16) extending along a central axis of the pen body (2), the piston rod (16) comprising a helical thread (19) on its outer surface;

a drive cylinder (24) mounted around the piston rod (16) within the pen body (2) so as to be capable of rotation about the axis but not translation along the axis;

a dose delivery tube (30) mounted concentrically around the drive cylinder (24), wherein an inner surface of the dose delivery tube (30) and an outer surface of the drive cylinder (24) are configured to form a mutually engaging non-locking thread (29,31); a manual dose selector (4) mounted concentrically around the dose delivery tube (30) so as to be capable of translation along the axis relative to the pen body (2) but not capable of rotation about the axis, wherein the manual dose selector (4) engages the dose delivery tube (30) such that moving the dose selector (4) axially compels the dose delivery tube (30) to move axially;

an activation button (6), wherein by pressing the activation button (6) the nature of the engagement between the manual dose selector (4) and the dose delivery tube (30) is changed from a dose setting mode in which the dose delivery tube (30) is permitted to rotate as it moves axially to a dose delivery mode in which the dose delivery tube (30) is not permitted to rotate as it moves axially; and

a piston guide (20,60) fixed relative to the pen body (2);

wherein one of the drive cylinder (24) and the piston guide (20,60) comprises an inner helical thread (25,58), which engages the thread (19) on the outer surface of the piston rod (16); and

wherein the other of the drive cylinder (24) and the piston guide (20,60) comprises a restraint (22,56), which engages the piston rod (16) to prevent relative rotation between the restraint (22,56) and the piston rod (16).

2. An injector pen according to claim 1, wherein the piston rod (16) comprises a longitudinal keyway (18) and the restraint (22,56) is a key in engagement with the keyway (18).

3. An injector pen according to claim 1 or claim 2, further comprising a ratchet (26) that acts between the pen body (2) and the drive cylinder (24) to permit only one direction of rotation of the drive cylinder (24) relative to the pen body (2).

4. An injector pen according to any preceding claim, wherein the pen body (2) comprises a generally cylindrical outer wall (37) and a concentric, cylindrical inner wall (38) spaced from the outer wall (37), the inner wall (38) being radially located between two of the movable components (4,24,30) of the pen.

5. An injector pen according to claim 4, wherein the inner wall (38) of the pen body (2) is radially located between the drive cylinder (24) and the dose delivery tube (30) in a region where the inner wall (38) does not interfere with the engagement of the non-locking thread (29,31).

6. An injector pen according to any preceding claim, in which the activation button (6) is located at a first end of the pen, whereby pressing the activation button (6) both selects the dose delivery mode and urges the dose delivery tube (30) to move axially and without rotation towards a second end of the pen, the axial movement of the dose delivery tube (30) acting through the non-locking thread (29,31 ) to cause the drive cylinder (24) to rotate, the drive cylinder (24) in turn acting through the restraint (22,56) and the helical thread (25,58) to drive the piston rod (16) in a direction from the first end of the pen towards the second end of the pen.

7. An injector pen according to claim 6, further incorporating a last dose lock-out mechanism comprising:

a cam (78) at or near the first end of the piston rod (16);

a deformable stop member (70) on the drive cylinder (24) positioned for engagement by the cam (78) as the piston rod (16) approaches the end of its movement towards the second end of the pen, whereby continued movement of the piston rod (16) deforms the stop member (70) radially outwards; and locking means (76) on the dose delivery tube (30) for engagement by the stop member (70) as it is deformed radially outwards, thereby to prevent relative rotation between the drive cylinder (24) and the dose delivery tube (30) in the dose setting mode.

8. An injector pen according to claim 7, wherein the locking means comprises a series of teeth (76) arranged circumferentially around the inner surface of the dose delivery tube (30).

9. An injector pen according to claim 8, wherein the stop member (70) and the teeth (76) form a ratchet that generates an audible signal during relative rotation between the drive cylinder (24) and the dose delivery tube (30) in the dose delivery mode.

10. An injector pen according to any of claims 6 to 9, further comprising:

means (9) for attaching a drug cartridge (8) to the second end of the pen for activation by the piston rod (16);

a piston reset element (46) comprising the piston guide (20), the piston reset element (46) being mounted around the axis at the second end of the pen and free to rotate relative to the pen body (2) when no drug cartridge (8) is attached; and

a reset brake (44), which locks the piston reset element (46) against rotation relative to the pen body (2) when a drug cartridge (8) is attached.

11. An injector pen according to any of claims 6 to 9, wherein the piston guide (60) comprises the said inner helical thread (58) and the drive cylinder (24) comprises the said restraint (56), the injector pen further comprising:

a piston reset element (64) mounted around the axis at the second end of the pen, wherein the piston guide (60) is formed by two or more deformable tabs (66) of the piston reset element (64), the tabs (66) having portions of the inner helical thread (58) on their respective inner surfaces, whereby when a drug cartridge (8) is attached to the pen the tabs (66) are deformed radially inwards to engage the helical thread (19) of the piston rod (16) but when no drug cartridge (8) is attached to the pen the tabs (66) do not engage the piston rod (16).

12. A method of operating an injector pen comprising:

in a dose setting mode, manually sliding a dose selector (4) axially and without rotation relative to a body (2) of the pen to an axial position that determines the dose to be delivered by the pen;

pressing an activation button (6) at a first end of the pen both to select a dose delivery mode and to urge the dose selector (4) to move axially and without rotation towards a second end of the pen, the axial movement of the dose selector (4) causing in the dose delivery mode axial but not rotational movement of a dose delivery tube (30) mounted concentrically within the dose selector (4), the axial movement of the dose delivery tube (30) acting through a non-locking thread (29,31) to cause rotation but not axial movement of a drive cylinder (24) mounted concentrically within the dose delivery tube (30), the rotation of the drive cylinder (24) in turn acting to drive a piston rod (16) along the axis of the injector pen in a direction from the first end of the pen towards the second end of the pen.

13. A method of operating an injector pen according to claim 12, wherein:

the piston rod (16) comprises a helical thread (19) on its outer surface;

the drive cylinder (24) comprises an inner helical thread (25), which engages the thread (19) on the outer surface of the piston rod (16); and

a piston guide (20) fixed relative to the pen body (2) comprises a restraint (22), which engages a non-circular section (18) of the piston rod (16) to prevent relative rotation between the piston guide (20) and the piston rod (16);

whereby the rotating inner helical thread (25) on the drive cylinder (24) acts on the non-rotating outer helical thread (19) of the piston rod (16) to drive the piston rod (16) along the axis of the injector pen.

14. A method of operating an injector pen according to claim 12, wherein:

the piston rod (16) comprises a helical thread (19) on its outer surface; a piston guide (60) fixed relative to the pen body (2) comprises an inner helical thread (58), which engages the thread (19) on the outer surface of the piston rod (16); and

the drive cylinder (24) comprises a restraint (56), which prevents relative rotation between the drive cylinder (24) and the piston rod (16);

whereby the rotation of the restraint (56) on the drive cylinder (24) forces rotation of the piston rod (16), and the rotating outer helical thread (19) on the piston rod (16) acts on the non-rotating inner helical thread (58) on the piston guide (60) to drive the piston rod (16) along the axis of the injector pen.

15. A method of operating an injector pen according to claim 14, further comprising:

before the first operation of the pen in dose setting mode, attaching a drug cartridge (8) to the second end of the pen, thereby to deform the piston guide (60) into threaded engagement with the piston rod (16);

after the last operation of the pen in dose delivery mode, removing the drug cartridge (8) from the second end of the pen, thereby to disengage the piston guide (60) from the piston rod (16);

then pushing the piston rod (16) axially back towards the first end of the pen.

16. A method of operating an injector pen according to any of claims 12 to 15, wherein in the dose setting mode the dose delivery tube (30) is constrained to move axially with the manual dose selector (4) but is permitted to rotate relative to the dose selector (30) and relative to the drive cylinder (24).

17. A method of operating an injector pen according to claim 16, wherein, during the dose delivery mode, a cam (78) at or near the first end of the piston rod (16) engages a deformable stop member (70) on the drive cylinder (24) as the piston rod (16) moves towards the second end of the pen, then continued movement of the piston rod (16) deforms the stop member (70) radially outwards to engage locking means (76) on the dose delivery tube (30), thereby to prevent relative rotation between the drive cylinder (24) and the dose delivery tube (30) in the dose setting mode.

18. A method of operating an injector pen according to claim 17, wherein the engagement between the stop member (70) and the locking means (76) during their relative rotation in the dose delivery mode produces an audible warning that the pen is locked out from setting a further dose.

19. A method of operating an injector pen according to any of claims 12 to 18, further comprising:

before the first operation of the pen in dose setting mode, attaching a drug cartridge (8) to the second end of the pen, thereby to engage a reset brake (44) to lock a piston reset element (46) against rotation relative to the pen body (2);

after the last operation of the pen in dose delivery mode, removing the drug cartridge (8) from the second end of the pen, thereby to disengage the reset brake (44) and permit rotation of the piston reset element (46) relative to the pen body (2);

then rotating the piston reset element (46) to drive the piston rod (16) back towards the first end of the pen.

20. An injector pen comprising:

a pen body (2);

a piston rod (16) extending along a central axis of the pen body (2);

a drivecylinder (24) within the pen body for driving movement of the piston rod (16) along the axis in a direction from a first end of the pen towards a second end of the pen;

means (9) for attaching a drug cartridge (8) to the second end of the pen so that the drug cartridge (8) may be operated by the movement of the piston rod (16);

a reset brake (44,64) mounted around the axis at the second end of the pen for actuation by attachment of a drug cartridge (8) to the pen, whereby:

when a drug cartridge (8) is attached the reset brake (44,64) constrains axial movement of the piston rod (16) to occur only in conjunction with movement of the drive cylinder (24); and when no drug cartridge is attached the reset brake (44,64) permits axial movement of the piston rod (16) to occur without movement of the drive cylinder (24).

21. An injector pen according to claim 20, wherein the reset brake (44,64) comprises a plurality of resilient arms (48,66) that are capable of deformation towards the axis to actuate the reset brake (44,64) when a drug cartridge (8) is attached to the pen.

22. An injector pen according to claim 21, wherein each arm (48,66) of the reset brake (44,64) comprises a tab that protrudes through an opening in the pen body (2) to a location where the tab can be engaged by a drug cartridge (8) upon attachment of the drug cartridge (8) to the pen.

23. An injector pen according to claim 21 or claim 22, wherein:

the drive cylinder (24) engages the piston rod (16) so as to prevent relative rotation therebetween;

the piston rod (16) comprises a helical thread (19) on its outer surface; and the reset brake (64) is fixed relative to the pen body (2) and comprises an inner helical thread (58), which engages the thread (19) on the outer surface of the piston rod (16) when a drug cartridge (8) is attached to the pen and does not engage the thread (19) on the outer surface of the piston rod (16) when no drug cartridge (8) is attached to the pen.

24. An injector pen according to claim 21 or claim 22, wherein:

the piston rod (16) comprises a helical thread (19) on its outer surface;

the drive cylinder (24) comprises an inner helical thread (25), which engages the thread (19) on the outer surface of the piston rod (16);

the injector pen further comprises a piston guide (20) mounted around the axis at the second end of the pen, the piston guide (20) engaging the piston rod (16) to prevent relative rotation between the piston guide (20) and the piston rod (16);

the reset brake (44) is fixed relative to the pen body; and the reset brake (44) engages a piston reset element (46) comprising the piston guide (20) to prevent rotation of the piston guide (20) when a drug cartridge (8) is attached to the pen and releases the piston reset element (46) to permit rotation of the piston guide (20) when no drug cartridge (8) is attached to the pen.

25. An injector pen according to claim 24, wherein a set of teeth (50) on the reset brake (44) engages a set of teeth (52) on the piston reset element (46) to prevent rotation of the piston guide (20) when a drug cartridge (8) is attached to the pen.

26. An injector pen according to claim 24 or 25, wherein the piston rod (16) comprises a longitudinal keyway (18) and the piston guide (20) comprises a key (22) in engagement with the keyway (18) to prevent relative rotation between the piston guide (20) and the piston rod (16).

27. An injector pen according to any of claims 20 to 26, further comprising a ratchet (26) that acts between the pen body (2) and the drive means (24) to permit the drive cylinder (24) to rotate in only one direction relative to the pen body (2).

28. A method of resetting an injector pen, the pen comprising a pen body (2), a piston rod (16), a drive cylinder (24) for driving movement of the piston rod (16) along an axis of the pen in a direction from a first end of the pen towards a second end of the pen, and a reset brake (44,64); the resetting method comprising:

detaching a drug cartridge (8) from the pen, thereby disengaging the reset brake (44,64) to permit axial movement of the piston rod (16) without movement of the drive cylinder (24);

manually moving the piston rod (16) in a direction towards the first end of the pen; and

attaching a further drug cartridge (8) to the pen, thereby engaging the reset brake (44,64) to constrain axial movement of the piston rod (16) to occur only in conjunction with movement of the drive cylinder (24).

29. A method according to claim 28, wherein the step of engaging the reset brake (44,64) comprises deforming a plurality of resilient arms of the reset brake (44,64) towards the axis.

30. A method according to claim 28 or claim 29, wherein:

the drive cylinder (24) engages the piston rod (16) so as to prevent relative rotation therebetween; and

the step of manually moving the piston rod (16) towards the first end of the pen comprises sliding the piston rod (16) along the axis without rotation.

31. A method according to claim 28 or claim 29, wherein:

an inner helical thread (25) on the drive cylinder (24) engages an outer helical thread (19) on the piston rod (16); and

the step of manually moving the piston rod (16) towards the first end of the pen comprises rotating the piston rod (16) about the axis to follow the helical thread (25) of the drive cylinder (24).